maintainer-scripts: Speed up git clone in gcc_release

When doing the 8.4-rc1, I've noticed (probably also because of the dying
disk on sourceware) that git clone is extremely slow, and furthermore when
all of us have some local snapshots, it is a waste of resources to download
everything again.  Especially for the -f runs when we'll need to wait until
git tag -s asks us for a gpg password interactively.

The following patch adds an option through which one can point the script
at a local gcc .git directory from which it can --dissociate --reference ...
during cloning to speed it up.

2020-02-27  Jakub Jelinek  <jakub@redhat.com>

	* gcc_release: Add support for -b local-git-repo argument.