scripts: Fix git-checkout.sh to work with branches other than master

scripts: Fix git-checkout.sh to work with branches other than master

Xen uses master for QEMU_UPSTREAM_REVISION, and has done for other trees too
in the path.  Apparently we've never specified a different branch, because the
git-clone rune only pulls in the master branch; it does not pull in diverging
branches.  Fix this by stating which branch/tag is wanted.

$TAG is really a committish, and git-clone's -b/--branch takes a committish
too.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>