contrib: Don't add push rules for personal and vendor spaces.

contrib: Don't add push rules for personal and vendor spaces.

Originally, it seemed like a good idea to add automatic 'push' rules
to the git configuration, so that personal- and vendor-space commits
would automatically push to the right place.  Unfortunately, this
changes git's behaviour and with these settings "git push" will try to
push all branches in a local tree up to the corresponding location on
the server (ignoring the push.default setting).  The only known
mitigation for this is to ALWAYS use "git push <server> <branch>".

So instead, we no-longer add those rules by default and will document
the options on the wiki.  We don't automatically remove the push
entries but do print out the command that will do so, if the user so
wishes.

	* gcc-git-customization.sh: Explain why we want the user's
	upstream account name.  Don't add push rules.  Check if push rules
	have been added and suggest that they should be removed.
	* git-fetch-vendor.sh: Don't add push rules.