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+v0.1, May 2005
+git-read-tree - Reads tree information into the directory cache
+'git-read-tree' (<tree-ish> | -m <tree-ish1> [<tree-ish2> <tree-ish3>])"
+Reads the tree information given by <tree> into the directory cache,
+but does not actually _update_ any of the files it "caches". (see:
+Optionally, it can merge a tree into the cache or perform a 3-way
+Trivial merges are done by "git-read-tree" itself. Only conflicting paths
+will be in unmerged state when "git-read-tree" returns.
+ Perform a merge, not just a read
+ The id of the tree object(s) to be read/merged.
+If '-m' is specified, "git-read-tree" performs 2 kinds of merge, a single tree
+merge if only 1 tree is given or a 3-way merge if 3 trees are
+Single Tree Merge
+If only 1 tree is specified, git-read-tree operates as if the user did not
+specify '-m', except that if the original cache has an entry for a
+given pathname; and the contents of the path matches with the tree
+being read, the stat info from the cache is used. (In other words, the
+cache's stat()s take precedence over the merged tree's)
+That means that if you do a "git-read-tree -m <newtree>" followed by a
+"git-checkout-cache -f -a", the "git-checkout-cache" only checks out
+the stuff that really changed.
+This is used to avoid unnecessary false hits when "git-diff-files" is
+run after git-read-tree.
+3-Way Merge
+Each "index" entry has two bits worth of "stage" state. stage 0 is the
+normal one, and is the only one you'd see in any kind of normal use.
+However, when you do "git-read-tree" with three trees, the "stage"
+starts out at 1.
+This means that you can do
+ git-read-tree -m <tree1> <tree2> <tree3>
+and you will end up with an index with all of the <tree1> entries in
+"stage1", all of the <tree2> entries in "stage2" and all of the
+<tree3> entries in "stage3".
+Furthermore, "git-read-tree" has special-case logic that says: if you see
+a file that matches in all respects in the following states, it
+"collapses" back to "stage0":
+ - stage 2 and 3 are the same; take one or the other (it makes no
+ difference - the same work has been done on stage 2 and 3)
+ - stage 1 and stage 2 are the same and stage 3 is different; take
+ stage 3 (some work has been done on stage 3)
+ - stage 1 and stage 3 are the same and stage 2 is different take
+ stage 2 (some work has been done on stage 2)
+The "git-write-tree" command refuses to write a nonsensical tree, and it
+will complain about unmerged entries if it sees a single entry that is not
+stage 0.
+Ok, this all sounds like a collection of totally nonsensical rules,
+but it's actually exactly what you want in order to do a fast
+merge. The different stages represent the "result tree" (stage 0, aka
+"merged"), the original tree (stage 1, aka "orig"), and the two trees
+you are trying to merge (stage 2 and 3 respectively).
+In fact, the way "git-read-tree" works, it's entirely agnostic about how
+you assign the stages, and you could really assign them any which way,
+and the above is just a suggested way to do it (except since
+"git-write-tree" refuses to write anything but stage0 entries, it makes
+sense to always consider stage 0 to be the "full merge" state).
+So what happens? Try it out. Select the original tree, and two trees
+to merge, and look how it works:
+- if a file exists in identical format in all three trees, it will
+ automatically collapse to "merged" state by the new git-read-tree.
+- a file that has _any_ difference what-so-ever in the three trees
+ will stay as separate entries in the index. It's up to "script
+ policy" to determine how to remove the non-0 stages, and insert a
+ merged version. But since the index is always sorted, they're easy
+ to find: they'll be clustered together.
+- the index file saves and restores with all this information, so you
+ can merge things incrementally, but as long as it has entries in
+ stages 1/2/3 (ie "unmerged entries") you can't write the result. So
+ now the merge algorithm ends up being really simple:
+ * you walk the index in order, and ignore all entries of stage 0,
+ since they've already been done.
+ * if you find a "stage1", but no matching "stage2" or "stage3", you
+ know it's been removed from both trees (it only existed in the
+ original tree), and you remove that entry.
+ * if you find a matching "stage2" and "stage3" tree, you remove one
+ of them, and turn the other into a "stage0" entry. Remove any
+ matching "stage1" entry if it exists too. .. all the normal
+ trivial rules ..
+Incidentally - it also means that you don't even have to have a
+separate subdirectory for this. All the information literally is in
+the index file, which is a temporary thing anyway. There is no need to
+worry about what is in the working directory, since it is never shown
+and never used.
+see also: link:git-write-tree.html[git-write-tree], link:git-ls-files.html[git-ls-files]
+Written by Linus Torvalds <>
+Documentation by David Greaves, Junio C Hamano and the git-list <>.
+Part of the link:git.html[git] suite