merge-base: add --merge-child option

This is helpful when examining branches with disjoint roots, for example because one is periodically merged into a subtree of the other.

With the --merge-child option, "git merge-base" will print a first-parent ancestor of the first revision given, where the commit printed is either a merge-base of the supplied revisions or a merge for which one of its parents (not the first) is a merge-base.

For example, given the history:

        A---C---G
             \
        B-----D---F
         \
          E

we have:

        $ git merge-base F E
        B

        $ git merge-base --merge-child F E
        D

	$ git merge-base F G
	C

	$ git merge-base --merge-child F G
	C

        $ git log --left-right F...E
        < F
        < D
        < C
        < A
        > E

        $ git log --left-right F...E --not $(git merge-base --merge-child F E)
        < F
        > E

The git-log case is useful because it allows us to limit the range of commits that we are examining for patch-identical changes when using --cherry. For example with git-gui in git.git I know that anything before the last merge of git-gui is not interesting:

        $ time git log --cherry master...git-gui/master >/dev/null
        real    0m32.731s
        user    0m31.956s
        sys     0m0.664s

        $ time git log --cherry master...git-gui/master --not \
                $(git merge-base --merge-child master git-gui/master) \
                >/dev/null
        real    0m2.296s
        user    0m2.193s
        sys     0m0.092s

The first commit is a small prerequisite to extract a useful function from builtin/tag.c to commit.c. The second is the main change (the commit message is identical to the text before this paragraph).

I'm not convinced that '--merge-child' is the right name for this but I think the functionality itself is useful.

John Keeping (2):
commit: add commit_list_contains function
merge-base: add --merge-child option

 Documentation/git-merge-base.txt |  6 ++++
 builtin/merge-base.c             | 61 ++++++++++++++++++++++++++++++++++++++--
 builtin/tag.c                    | 10 +------
 commit.c                         |  8 ++++++
 commit.h                         |  1 +
 t/t6010-merge-base.sh            | 25 ++++++++++++++--
 6 files changed, 98 insertions(+), 13 deletions(-)

1.8.3.rc1.289.gcb3647f