Pass empty file to p4merge where no base is suitable.

Pass empty file to p4merge where no base is suitable.

Modify the p4merge client command to pass a reference to an empty file
instead of the local file when no base revision available.

In the situation where a merge tries to add a file from one branch
into a branch that already contains that file (by name), p4merge
currently seems to have successfully automatically resolved the
'conflict' when it is opened (correctly if the files differed by
just whitespace for example) but leaves the save button disabled. This
means the user of the p4merge client cannot commit the resolved
changes back to disk and merely exits, leaving the original
(merge-conflicted) file intact on the disk.

Provide an empty base file to p4merge so that it leaves the save
button enabled.  This will allow saving of the auto-resolution to
disk.

Signed-off-by: Ciaran Jessup <ciaranj@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>