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...