xdiff-merge: optionally show conflicts in "diff3 -m" style

xdiff-merge: optionally show conflicts in "diff3 -m" style

When showing conflicting merges, we traditionally followed RCS's merge
output format.  The output shows:

 <<<<<<<
 postimage from one side;
 =======
 postimage of the other side; and
 >>>>>>>

Some poeple find it easier to be able to understand what is going on when
they can view the common ancestor's version, which is used by "diff3 -m",
which shows:

 <<<<<<<
 postimage from one side;
 |||||||
 shared preimage;
 =======
 postimage of the other side; and
 >>>>>>>

This is an initial step to bring that as an optional feature to git.
Only "git merge-file" has been converted, with "--diff3" option.

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