contrib: Change 'remote' for personal branches and add branch creation script

Whilst trying to convert the add vendor branch script to work with
personal branches I encountered a minor issue where git would report
ambiguous refs when checking out the new branch.

It turns out that this is because git considers <me>/<topic> to be
ambiguous if both

  refs/heads/<me>/<topic>

and

  refs/remotes/<me>/<topic>

exist in the list of known branches.

Having thought about this a bit, I think the best solution is to adopt
something more like the vendors space and call the remote users/<me>
(this also works better if you want to set up remotes to track other
users branches as well).

So this patch has two parts.

1) It updates gcc-git-customization.sh to set up the new 'remote' and
converts any existing remote and branches tracking that to the new
format
2) It adds a new script to set up a personal branch on the gcc git repository.

	* gcc-git-customization.sh: Use users/<pfx> for the personal remote
	rather than just <pfx>.  Convert any existing personal branches to the
	new remote.
	* git-add-user-branch.sh: New file.