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+A revision parameter typically, but not necessarily, names a
+commit object. They use what is called an 'extended SHA1'
+syntax. Here are various ways to spell object names. The
+ones listed near the end of this list are to name trees and
+blobs contained in a commit.
+* The full SHA1 object name (40-byte hexadecimal string), or
+ a substring of such that is unique within the repository.
+ E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
+ name the same commit object if there are no other object in
+ your repository whose object name starts with dae86e.
+* An output from 'git describe'; i.e. a closest tag, optionally
+ followed by a dash and a number of commits, followed by a dash, a
+ `g`, and an abbreviated object name.
+* A symbolic ref name. E.g. 'master' typically means the commit
+ object referenced by refs/heads/master. If you
+ happen to have both heads/master and tags/master, you can
+ explicitly say 'heads/master' to tell git which one you mean.
+ When ambiguous, a `<name>` is disambiguated by taking the
+ first match in the following rules:
+ . if `$GIT_DIR/<name>` exists, that is what you mean (this is usually
+ useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD` and `MERGE_HEAD`);
+ . otherwise, `refs/<name>` if exists;
+ . otherwise, `refs/tags/<name>` if exists;
+ . otherwise, `refs/heads/<name>` if exists;
+ . otherwise, `refs/remotes/<name>` if exists;
+ . otherwise, `refs/remotes/<name>/HEAD` if exists.
+HEAD names the commit your changes in the working tree is based on.
+FETCH_HEAD records the branch you fetched from a remote repository
+with your last 'git fetch' invocation.
+ORIG_HEAD is created by commands that moves your HEAD in a drastic
+way, to record the position of the HEAD before their operation, so that
+you can change the tip of the branch back to the state before you ran
+them easily.
+MERGE_HEAD records the commit(s) you are merging into your branch
+when you run 'git merge'.
+Note that any of the `refs/*` cases above may come either from
+the `$GIT_DIR/refs` directory or from the `$GIT_DIR/packed-refs` file.
+* A ref followed by the suffix '@' with a date specification
+ enclosed in a brace
+ pair (e.g. '\{yesterday\}', '\{1 month 2 weeks 3 days 1 hour 1
+ second ago\}' or '\{1979-02-26 18:30:00\}') to specify the value
+ of the ref at a prior point in time. This suffix may only be
+ used immediately following a ref name and the ref must have an
+ existing log ($GIT_DIR/logs/<ref>). Note that this looks up the state
+ of your *local* ref at a given time; e.g., what was in your local
+ `master` branch last week. If you want to look at commits made during
+ certain times, see `--since` and `--until`.
+* A ref followed by the suffix '@' with an ordinal specification
+ enclosed in a brace pair (e.g. '\{1\}', '\{15\}') to specify
+ the n-th prior value of that ref. For example 'master@\{1\}'
+ is the immediate prior value of 'master' while 'master@\{5\}'
+ is the 5th prior value of 'master'. This suffix may only be used
+ immediately following a ref name and the ref must have an existing
+ log ($GIT_DIR/logs/<ref>).
+* You can use the '@' construct with an empty ref part to get at a
+ reflog of the current branch. For example, if you are on the
+ branch 'blabla', then '@\{1\}' means the same as 'blabla@\{1\}'.
+* The special construct '@\{-<n>\}' means the <n>th branch checked out
+ before the current one.
+* The suffix '@\{upstream\}' to a ref (short form 'ref@\{u\}') refers to
+ the branch the ref is set to build on top of. Missing ref defaults
+ to the current branch.
+* A suffix '{caret}' to a revision parameter (e.g. 'HEAD{caret}') means the first parent of
+ that commit object. '{caret}<n>' means the <n>th parent (i.e.
+ 'rev{caret}'
+ is equivalent to 'rev{caret}1'). As a special rule,
+ 'rev{caret}0' means the commit itself and is used when 'rev' is the
+ object name of a tag object that refers to a commit object.
+* A suffix '{tilde}<n>' to a revision parameter means the commit
+ object that is the <n>th generation grand-parent of the named
+ commit object, following only the first parent. I.e. rev~3 is
+ equivalent to rev{caret}{caret}{caret} which is equivalent to
+ rev{caret}1{caret}1{caret}1. See below for a illustration of
+ the usage of this form.
+* A suffix '{caret}' followed by an object type name enclosed in
+ brace pair (e.g. `v0.99.8{caret}\{commit\}`) means the object
+ could be a tag, and dereference the tag recursively until an
+ object of that type is found or the object cannot be
+ dereferenced anymore (in which case, barf). `rev{caret}0`
+ introduced earlier is a short-hand for `rev{caret}\{commit\}`.
+* A suffix '{caret}' followed by an empty brace pair
+ (e.g. `v0.99.8{caret}\{\}`) means the object could be a tag,
+ and dereference the tag recursively until a non-tag object is
+ found.
+* A colon, followed by a slash, followed by a text (e.g. `:/fix nasty bug`): this names
+ a commit whose commit message starts with the specified text.
+ This name returns the youngest matching commit which is
+ reachable from any ref. If the commit message starts with a
+ '!', you have to repeat that; the special sequence ':/!',
+ followed by something else than '!' is reserved for now.
+* A suffix ':' followed by a path (e.g. `HEAD:README`); this names the blob or tree
+ at the given path in the tree-ish object named by the part
+ before the colon.
+ ':path' (with an empty part before the colon, e.g. `:README`)
+ is a special case of the syntax described next: content
+ recorded in the index at the given path.
+* A colon, optionally followed by a stage number (0 to 3) and a
+ colon, followed by a path (e.g. `:0:README`); this names a blob object in the
+ index at the given path. Missing stage number (and the colon
+ that follows it, e.g. `:README`) names a stage 0 entry. During a merge, stage
+ 1 is the common ancestor, stage 2 is the target branch's version
+ (typically the current branch), and stage 3 is the version from
+ the branch being merged.
+Here is an illustration, by Jon Loeliger. Both commit nodes B
+and C are parents of commit node A. Parent commits are ordered
+G H I J
+ \ / \ /
+ D E F
+ \ | / \
+ \ | / |
+ \|/ |
+ B C
+ \ /
+ \ /
+ A
+ A = = A^0
+ B = A^ = A^1 = A~1
+ C = A^2 = A^2
+ D = A^^ = A^1^1 = A~2
+ E = B^2 = A^^2
+ F = B^3 = A^^3
+ G = A^^^ = A^1^1^1 = A~3
+ H = D^2 = B^^2 = A^^^2 = A~2^2
+ I = F^ = B^3^ = A^^3^
+ J = F^2 = B^3^2 = A^^3^2
+History traversing commands such as 'git log' operate on a set
+of commits, not just a single commit. To these commands,
+specifying a single revision with the notation described in the
+previous section means the set of commits reachable from that
+commit, following the commit ancestry chain.
+To exclude commits reachable from a commit, a prefix `{caret}`
+notation is used. E.g. `{caret}r1 r2` means commits reachable
+from `r2` but exclude the ones reachable from `r1`.
+This set operation appears so often that there is a shorthand
+for it. When you have two commits `r1` and `r2` (named according
+to the syntax explained in SPECIFYING REVISIONS above), you can ask
+for commits that are reachable from r2 excluding those that are reachable
+from r1 by `{caret}r1 r2` and it can be written as `r1..r2`.
+A similar notation `r1\...r2` is called symmetric difference
+of `r1` and `r2` and is defined as
+`r1 r2 --not $(git merge-base --all r1 r2)`.
+It is the set of commits that are reachable from either one of
+`r1` or `r2` but not from both.
+Two other shorthands for naming a set that is formed by a commit
+and its parent commits exist. The `r1{caret}@` notation means all
+parents of `r1`. `r1{caret}!` includes commit `r1` but excludes
+all of its parents.
+Here are a handful of examples:
+ D G H D
+ D F G H I J D F
+ ^G D H D
+ ^D B E I J F B
+ B...C G H D E B C
+ ^D B C E I J F B C
+ C^@ I J F
+ F^! D G H D F