merge-recursive: provide a better label for diff3 common ancestor

merge-recursive: provide a better label for diff3 common ancestor

In commit 7ca56aa07619 ("merge-recursive: add a label for ancestor",
2010-03-20), a label was added for the '||||||' line to make it have
the more informative heading '|||||| merged common ancestors', with
the statement:

    It would be nicer to use a more informative label.  Perhaps someone
    will provide one some day.

This chosen label was perfectly reasonable when recursiveness kicks in,
i.e. when there are multiple merge bases.  (I can't think of a better
label in such cases.)  But it is actually somewhat misleading when there
is a unique merge base or no merge base.  Change this based on the
number of merge bases:
    >=2: "merged common ancestors"
    1:   <abbreviated commit hash>
    0:   "<empty tree>"

Tests have also been added to check that we get the right ancestor name
for each of the three cases.

Also, since merge_recursive() and merge_trees() have polar opposite
pre-conditions for opt->ancestor, document merge_recursive()'s
pre-condition with an assertion.  (An assertion was added to
merge_trees() already a few commits ago.)  The differences in
pre-conditions stem from two factors: (1) merge_trees() does not recurse
and thus does not have multiple sub-merges to worry about -- each of
which would require a different value for opt->ancestor, (2)
merge_trees() is only passed trees rather than commits and thus cannot
internally guess as good of a label.  Thus, while external callers of
merge_trees() are required to provide a non-NULL opt->ancestor,
merge_recursive() expects to set this value itself.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>