contrib/gcc-changelog: Check whether revert-commit exists

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (GitCommit.__init__):
	Handle commit_to_info_hook = None; otherwise, if None,
	regard it as error.
	(to_changelog_entries): Handle commit_to_info_hook = None;
	if info is None, create a warning for it.
	* gcc-changelog/git_email.py (GitEmail.__init__):
	call super() with commit_to_info_hook=None instead
	of a lamda function.