.gitattributes: set file type for C files

.gitattributes: set file type for C files

Set the diff attribute for C source file to "cpp" in order to improve
git's ability to determine hunk headers.  In particular it helps avoid
showing unindented labels in hunk headers.  That in turn is useful for
git diff -W and git grep -W, which show whole functions now instead of
stopping at a label.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>