merge-ort: add new merge_ort_generic() function

merge-ort: add new merge_ort_generic() function

merge-recursive.[ch] have three entry points:
  * merge_trees()
  * merge_recursive()
  * merge_recursive_generic()
merge-ort*.[ch] only has equivalents for the first two.  Add an
equivalent for the final entry point, so we can switch callers to
use it and remove merge-recursive.[ch].

While porting it over, finally fix the issue with the label for the
ancestor (used when merge.conflictStyle=diff3 as a conflict label).
merge-recursive.c has traditionally not allowed callers to set that
label, but I have found that problematic for years.

(Side note: This function was initially part of the merge-ort rewrite,
but reviewers questioned the ancestor label funnyness which I was
never really happy with anyway.  It resulted in me jettisoning it and
hoping at the time that I would eventually be able to force the existing
callers to use some other API.  That worked with `git stash`, as per
874cf2a60444 (stash: apply stash using 'merge_ort_nonrecursive()',
2022-05-10), but this API is the most reasonable one for `git am` and
`git merge-recursive`, if we can just allow them some freedom over the
ancestor label.)

The merge_recursive_generic() function did not know whether it was being
invoked by `git stash`, `git merge-recursive`, or `git am`, and the
choice of meaningful ancestor label, when there is a unique ancestor,
varies for these different callers:

  * git am: ancestor is a constructed "fake ancestor" that user knows
            nothing about and has no access to.  (And is different than
            the normal thing we mean by a "virtual merge base" which is
            the merging of merge bases.)
  * git merge-recursive: ancestor might be a tree, but at least it
                         was one specified by the user (if they invoked
                         merge-recursive directly)
  * git stash: ancestor was the commit serving as the stash base

Thus, using a label like "constructed merge base" (as
merge_recursive_generic() does) presupposes that `git am` is the only
caller; it is incorrect for other callers.  This label has thrown me off
more than once.  Allow the caller to override when there is a unique
merge base.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>