Make build-many-glibcs.py clean git source directories.

Make build-many-glibcs.py clean git source directories.

With the git checkouts of Hurd components in build-many-glibcs.py
involving running autoreconf, there's a risk that generated files
could be left behind by an old autoreconf run (if an old version of
the sources generates those files in the source directory but a new
version does not).

This patch avoids that by using git clean -dxfq when updating git
checkouts.  In this patch, that's conditional on --replace-sources, to
avoid removing any local not-checked-in files someone may have in
their checkout unless the option has been specifically passed that
says it's OK to blow old checkouts away, complete with any local
changes to them.

	* scripts/build-many-glibcs.py (Context.git_checkout): Use git
	clean -dxfq for git updates when replacing sources.