patchtest: check for untracked changes

patchtest: check for untracked changes

[YOCTO #15243]

Avoid overwriting local changes when running patchtest by checking for
anything unstaged or uncommitted in the target repo, and logging an
error if something is found. This will provide the user helpful feedback
if (for example) they forgot to commit a change for their patch under
test, and will leave the target repository in a reasonable state (rather
than a temporary branch created by patchtest).

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>