git: Add `-f` flag to all checkouts

git: Add `-f` flag to all checkouts

All the current instances of 'git checkout' are meant to be
destructive (several have `git clean -ffdx` afterwards).  In cases
where a file is generated and untracked in one version of Xen but tracked
in another version of Xen, `git checkout` will fail without the `-f` flag.

Since we're going to be destructive anyway, add `-f` to all `git
checkout` commands.

Signed-off-by: Georg Dunlap <george.dunlap@citrix.com>