merge-recursive: Introduce a merge_file convenience function

merge-recursive: Introduce a merge_file convenience function

merge_file previously required diff_filespec arguments, but all callers
only had sha1s and modes.  Rename merge_file to merge_file_1 and introduce
a new merge_file convenience function which takes the sha1s and modes and
creates the temporary diff_filespec variables needed to call merge_file_1.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>