The primary use of conflict markers is to help the user who resolves
the final (outer) merge by hand to show which part came from which
branch by separating the blocks of lines apart.  When the conflicted
parts from a "virtual ancestor" merge created by merge-recursive
remains in the common ancestor part in the final result, however,
the conflict markers that are the same size as the final merge
become harder to see.

Increase the conflict marker size slightly for these inner merges so
that the markers from the final merge and cruft from internal merge
can be distinguished more easily.

This would help reduce the common issue that prevents "rerere" from
being used on a really complex conflict.

Signed-off-by: Junio C Hamano <gitster@pobox.com>