Add a .gitattributes file (#6381)

Currently this just sets the git diff driver for C source code files. Git can be told to apply language-specific rules when generating diffs. Enabling this for C source code files (*.c and *.h) means that function names are printed correctly. Specifically, doing so prevents "git diff" from mistakenly considering unindented goto labels as function names. This has the same effect as adding [diff "default"] xfuncname = "^[[:alpha:]$_].*[^:]$" to your git config file. e.g get @@ -10,7 +10,7 @@ int main(void) instead of @@ -10,7 +10,7 @@ again: Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>