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

This is the identical (except for a ChangeLog typo) to
  commit r14-3777-gff20bce9f5879878f352f1fcd6ade023a2067598

It reverts the test revert in commit
  r14-3778-gfbbd9001e9b6f2c59b542cc53a8f9183514091ce
which has a bogus commit hash and should have been rejected,
but we missed that - before testing - the script had to be
manually copied to the right place on sourceware to be
affective as pre-commit hook.

Thus, the r14-3777 commit had to be reinstate by this commit ...

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 lambda function.