merge-one-file: use empty- or common-base condintionally in two-stage merge.
merge-one-file: use empty- or common-base condintionally in two-stage merge.

If two sides added the same path completely different thing, it is
easier to see the merge pivoting on /dev/null.  So check the size of
the common section we have found, and empty it if it is too small.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git