merge-one-file: use common as base, instead of emptiness.

merge-one-file: use common as base, instead of emptiness.

Unlike the previous round that merged the path added differently
in each branches using emptiness as the base, compute a common
version and use it as input to 'merge' program.

This would show the resulting (still conflicting) file left in
the working tree as:

	common file contents...
	<<<<<< FILENAME
	version from our branch...
	======
	version from their branch...
	>>>>>> .merge_file_XXXXXX
	more common file contents...

when both sides added similar contents.

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