[PATCH] Document the trivial merge rules for 3(+more ancestors)-way merges.

Signed-off-by: Daniel Barkalow
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/technical/trivial-merge.txt

@@ -0,0 +1,92 @@
+Trivial merge rules
+===================
+
+This document describes the outcomes of the trivial merge logic in read-tree.
+
+One-way merge
+-------------
+
+This replaces the index with a different tree, keeping the stat info
+for entries that don't change, and allowing -u to make the minimum
+required changes to the working tree to have it match.
+
+   index   tree    result
+   -----------------------
+   *       (empty) (empty)
+   (empty) tree    tree
+   index+  tree    tree
+   index+  index   index+
+
+Two-way merge
+-------------
+
+
+
+Three-way merge
+---------------
+
+It is permitted for the index to lack an entry; this does not prevent
+any case from applying.
+
+If the index exists, it is an error for it not to match either the
+head or (if the merge is trivial) the result.
+
+If multiple cases apply, the one used is listed first.
+
+A result of "no merge" means that index is left in stage 0, ancest in
+stage 1, head in stage 2, and remote in stage 3 (if any of these are
+empty, no entry is left for that stage). Otherwise, the given entry is
+left in stage 0, and there are no other entries.
+
+A result of "no merge" is an error if the index is not empty and not
+up-to-date.
+
+*empty* means that the tree must not have a directory-file conflict
+ with the entry.
+
+For multiple ancestors or remotes, a '+' means that this case applies
+even if only one ancestor or remote fits; normally, all of the
+ancestors or remotes must be the same.
+
+case  ancest    head    remote    result
+----------------------------------------
+1     (empty)+  (empty) (empty)   (empty)
+2ALT  (empty)+  *empty* remote    remote
+2ALT  (empty)+  *empty* remote    remote
+2     (empty)^  (empty) remote    no merge
+3ALT  (empty)+  head    *empty*   head
+3     (empty)^  head    (empty)   no merge
+4     (empty)^  head    remote    no merge
+5ALT  *         head    head      head
+6     ancest^   (empty) (empty)   no merge
+8ALT  ancest    (empty) ancest    (empty)
+7     ancest+   (empty) remote    no merge
+9     ancest+   head    (empty)   no merge
+10ALT ancest    ancest  (empty)   (empty)
+11    ancest+   head    remote    no merge
+16    anc1/anc2 anc1    anc2      no merge
+13    ancest+   head    ancest    head
+14    ancest+   ancest  remote    remote
+14ALT ancest+   ancest  remote    remote
+
+Only #2ALT and #3ALT use *empty*, because these are the only cases
+where there can be conflicts that didn't exist before. Note that we
+allow directory-file conflicts between things in different stages
+after the trivial merge.
+
+A possible alternative for #6 is (empty), which would make it like
+#1. This is not used, due to the likelihood that it arises due to
+moving the file to multiple different locations or moving and deleting
+it in different branches.
+
+Case #1 is included for completeness, and also in case we decide to
+put on '+' markings; any path that is never mentioned at all isn't
+handled.
+
+Note that #16 is when both #13 and #14 apply; in this case, we refuse
+the trivial merge, because we can't tell from this data which is
+right. This is a case of a reverted patch (in some direction, maybe
+multiple times), and the right answer depends on looking at crossings
+of history or common ancestors of the ancestors.
+
+The status as of Sep 5 is that multiple remotes are not supported