contrib: Check and if needed set user.name and user.email in gcc-git-customization.sh

contrib: Check and if needed set user.name and user.email in gcc-git-customization.sh

As discussed on IRC, this adds a couple more checks in the
customization setup for git.  If the variables user.name and
user.email are not set anywhere in the git config hierarchy, we set
some local values.  We always ask about the values we detect and if
the user gives an answer that is new, we save that in the local
config: this gives the opportunity to use different values to those
configured for the global space.

Also cleaned up a couple of minor niggles, such as using $(cmd) rather
than `cmd` for subshells and some quoting issues when using eval.

	* gcc-git-customization.sh: Check that user.name and user.email
	are set.  Use $(cmd) instead of `cmd`.  Fix variable quoting when
	using eval.