contrib: Make prepare-commit-msg hook safe for older branches

contrib: Make prepare-commit-msg hook safe for older branches

If a user installs this script as .git/hooks/prepare-commit-msg and then
works on an old branch which doesn't have the mklog.py script, trying to
commit will fail with an error like:

environment: /.../gcc/contrib/mklog.py: No such file or directory

This makes it exit cleanly so it's possible to commit.

contrib/ChangeLog:

	* prepare-commit-msg: Do nothing if the mklog.py script isn't
	present.