.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.

---
 .gitattributes | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 5e98806..320e33c 100644
@@ -1,3 +1,3 @@
 * whitespace=!indent,trail,space
-*.[ch] whitespace=indent,trail,space
+*.[ch] whitespace=indent,trail,space diff=cpp
 *.sh whitespace=indent,trail,space

2.9.0