improve git diff

Sometimes git does not print the name of the function being changed
after @@. It happens especially after a goto label which is not indented.
Giving a hint about the languages of files .c, .h and .py
will improve hunk headers of "git diff" rendering.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>

.gitattributes

@@ -0,0 +1,3 @@
+*.c   diff=cpp
+*.h   diff=cpp
+*.py  diff=python