merge-recursive: Fix modify/delete resolution in the recursive case

When o->call_depth>0 and we have conflicts, we try to find "middle ground"
when creating the virtual merge base.  In the case of content conflicts,
this can be done by doing a three-way content merge and using the result.
In all parts where the three-way content merge is clean, it is the correct
middle ground, and in parts where it conflicts there is no middle ground
but the conflict markers provide a good compromise since they are unlikely
to accidentally match any further changes.

In the case of a modify/delete conflict, we cannot do the same thing.
Accepting either endpoint as the resolution for the virtual merge base
runs the risk that when handling the non-recursive case we will silently
accept one person's resolution over another without flagging a conflict.
In this case, the closest "middle ground" we have is actually the merge
base of the candidate merge bases.  (We could alternatively attempt a
three way content merge using an empty file in place of the deleted file,
but that seems to be more work than necessary.)

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

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