coccinelle: put sane filenames into output patches

Coccinelle outputs its suggested transformations as patches, whose
header looks something like this:

  --- commit.c
  +++ /tmp/cocci-output-19250-7ae78a-commit.c

Note the lack of 'diff --opts <old> <new>' line, the differing number
of path components on the --- and +++ lines, and the nonsensical
filename on the +++ line.  'patch -p0' can still apply these patches,
as it takes the filename to be modified from the --- line.  Alas, 'git
apply' can't, because it takes the filename from the +++ line, and
then complains about the nonexisting file.

Pass the '--patch .' options to Coccinelle via the SPATCH_FLAGS 'make'
variable, as it seems to make it generate proper context diff patches,
with the header starting with a 'diff ...' line and containing sane
filenames.  The resulting 'contrib/coccinelle/*.cocci.patch' files
then can be applied both with 'git apply' and 'patch' (even without
'-p0').

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -564,7 +564,7 @@ SPATCH = spatch
 export TCL_PATH TCLTK_PATH
 
 SPARSE_FLAGS =
-SPATCH_FLAGS = --all-includes
+SPATCH_FLAGS = --all-includes --patch .