ll_merge(): add ancestor label parameter for diff3-style output

ll_merge(): add ancestor label parameter for diff3-style output

Commands using the ll_merge() function will present conflict hunks
imitating ‘diff3 -m’ output if the merge.conflictstyle configuration
option is set appropriately.  Unlike ‘diff3 -m’, the output does not
include a label for the merge base on the ||||||| line of the output,
and some tools misparse the conflict hunks without that.

Add a new ancestor_label parameter to ll_merge() to give callers the
power to rectify this situation.  If ancestor_label is NULL, the output
format is unchanged.  All callers pass NULL for now.

Requested-by: Stefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>