git: add diff=cpp attribute for C/C++ source files

This improves diff hunk name detection. Needed for checkpatch to
correctly detect if patched code belongs to a function.

NO_DOC=git
NO_TEST=git
NO_CHANGELOG=git

.gitattributes

 *.snap binary
 *.xlog binary
+*.c diff=cpp
+*.h diff=cpp
+*.cc diff=cpp
+*.hh diff=cpp
+*.cpp diff=cpp
+*.hpp diff=cpp
+*.c++ diff=cpp
+*.h++ diff=cpp