Create a gitattributes file

When creating an archive, the GitHub repository and VCS-specific files
are now ignored.
The diff "hunk header" is customized for C/C++ [1].

[1]: https://git-scm.com/docs/gitattributes#_defining_a_custom_hunk_header

.gitattributes

@@ -0,0 +1,5 @@
+.git* export-ignore
+.hg* export-ignore
+*.c diff=cpp
+*.cpp diff=cpp
+*.h diff=cpp