SYNOPSIS
--------
-'git merge-base' [--all] <commit> <commit>
+'git merge-base' [--all] <commit> <commit>...
DESCRIPTION
-----------
-'git-merge-base' finds as good a common ancestor as possible between
-the two commits. That is, given two commits A and B, `git merge-base A
-B` will output a commit which is reachable from both A and B through
-the parent relationship.
+'git-merge-base' finds best common ancestor(s) between two commits to use
+in a three-way merge. One common ancestor is 'better' than another common
+ancestor if the latter is an ancestor of the former. A common ancestor
+that does not have any better common ancestor is a 'best common
+ancestor', i.e. a 'merge base'. Note that there can be more than one
+merge base for a pair of commits.
-Given a selection of equally good common ancestors it should not be
-relied on to decide in any particular way.
-
-The 'git-merge-base' algorithm is still in flux - use the source...
+Among the two commits to compute the merge base from, one is specified by
+the first commit argument on the command line; the other commit is a
+(possibly hypothetical) commit that is a merge across all the remaining
+commits on the command line. As the most common special case, specifying only
+two commits on the command line means computing the merge base between
+the given two commits.
OPTIONS
-------
--all::
- Output all common ancestors for the two commits instead of
- just one.
+ Output all merge bases for the commits, instead of just one.
+
+DISCUSSION
+----------
+
+Given two commits 'A' and 'B', `git merge-base A B` will output a commit
+which is reachable from both 'A' and 'B' through the parent relationship.
+
+For example, with this topology:
+
+ o---o---o---B
+ /
+ ---o---1---o---o---o---A
+
+the merge base between 'A' and 'B' is '1'.
+
+Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the
+merge base between 'A' and a hypothetical commit 'M', which is a merge
+between 'B' and 'C'. For example, with this topology:
+
+ o---o---o---o---C
+ /
+ / o---o---o---B
+ / /
+ ---2---1---o---o---o---A
+
+the result of `git merge-base A B C` is '1'. This is because the
+equivalent topology with a merge commit 'M' between 'B' and 'C' is:
+
+
+ o---o---o---o---o
+ / \
+ / o---o---o---o---M
+ / /
+ ---2---1---o---o---o---A
+
+and the result of `git merge-base A M` is '1'. Commit '2' is also a
+common ancestor between 'A' and 'M', but '1' is a better common ancestor,
+because '2' is an ancestor of '1'. Hence, '2' is not a merge base.
+
+When the history involves criss-cross merges, there can be more than one
+'best' common ancestor for two commits. For example, with this topology:
+
+ ---1---o---A
+ \ /
+ X
+ / \
+ ---2---o---o---B
+
+both '1' and '2' are merge-bases of A and B. Neither one is better than
+the other (both are 'best' merge bases). When the `--all` option is not given,
+it is unspecified which best one is output.
Author
------