Cumulative update of merge-recursive in C

This contains mainly three sorts of fixes:

- get rid of small wrapper functions
- reuse the diff_filespec structure when sha1, mode & path are needed
- Junio's pedantic updates
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>