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-rw-r--r--Documentation/git-read-tree.txt74
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diff --git a/Documentation/git-read-tree.txt b/Documentation/git-read-tree.txt
index cbde13d..6440c4b 100644
--- a/Documentation/git-read-tree.txt
+++ b/Documentation/git-read-tree.txt
@@ -53,6 +53,80 @@ the stuff that really changed.
This is used to avoid unnecessary false hits when "git-diff-files" is
run after git-read-tree.
+
+Two Tree Merge
+~~~~~~~~~~~~~~
+
+Typically, this is invoked as "git-read-tree -m $H $M", where $H
+is the head commit of the current repository, and $M is the head
+of a foreign tree, which is simply ahead of $H (i.e. we are in a
+fast forward situation).
+
+When two trees are specified, the user is telling git-read-tree
+the following:
+
+ (1) The current index and work tree is derived from $H, but
+ the user may have local changes in them since $H;
+
+ (2) The user wants to fast-forward to $M.
+
+In this case, the "git-read-tree -m $H $M" command makes sure
+that no local change is lost as the result of this "merge".
+Here are the "carry forward" rules:
+
+ I (index) H M Result
+ -------------------------------------------------------
+ 0 nothing nothing nothing (does not happen)
+ 1 nothing nothing exists use M
+ 2 nothing exists nothing remove path from cache
+ 3 nothing exists exists use M
+
+ clean I==H I==M
+ ------------------
+ 4 yes N/A N/A nothing nothing keep index
+ 5 no N/A N/A nothing nothing keep index
+
+ 6 yes N/A yes nothing exists keep index
+ 7 no N/A yes nothing exists keep index
+ 8 yes N/A no nothing exists fail
+ 9 no N/A no nothing exists fail
+
+ 10 yes yes N/A exists nothing remove path from cache
+ 11 no yes N/A exists nothing fail
+ 12 yes no N/A exists nothing fail
+ 13 no no N/A exists nothing fail
+
+ clean (H=M)
+ ------
+ 14 yes exists exists keep index
+ 15 no exists exists keep index
+
+ clean I==H I==M (H!=M)
+ ------------------
+ 16 yes no no exists exists fail
+ 17 no no no exists exists fail
+ 18 yes no yes exists exists keep index
+ 19 no no yes exists exists keep index
+ 20 yes yes no exists exists use M
+ 21 no yes no exists exists fail
+
+In all "keep index" cases, the cache entry stays as in the
+original index file. If the entry were not up to date,
+git-read-tree keeps the copy in the work tree intact when
+operating under the -u flag.
+
+When this form of git-read-tree returns successfully, you can
+see what "local changes" you made are carried forward by running
+"git-diff-cache --cached $M". Note that this does not
+necessarily match "git-diff-cache --cached $H" would have
+produced before such a two tree merge. This is because of cases
+18 and 19 --- if you already had the changes in $M (e.g. maybe
+you picked it up via e-mail in a patch form), "git-diff-cache
+--cached $H" would have told you about the change before this
+merge, but it would not show in "git-diff-cache --cached $M"
+output after two-tree merge.
+
+
3-Way Merge
~~~~~~~~~~~
Each "index" entry has two bits worth of "stage" state. stage 0 is the