Rewrite maintainer-scripts/update_version_git

Rewrite maintainer-scripts/update_version_git

This patch rewrites update_version_git to be just a thin wrapper around
Martin's new python script.  This just arranges to check out the gcc
repo in a temporary directory, copy out the contrib scripts so that
the running script doesn't change with branch checkouts and runs the script.

I've run it today manually but hopefully we can do it from cron again
from tomorrow.

2020-05-27  Jakub Jelinek  <jakub@redhat.com>

	* update_version_git: Rewrite using
	contrib/gcc-changelog/git_update_version.py.