git: Have git handle 'autostash' directly.

This addresses kdesrc-build bugs in local change detection.

@dfaure has noted continuing issues with kdesrc-build in trying to stash
what it believes are local changes, the stash being empty, and then
kdesrc-build throwing an error trying to restore the non-existent stash.

Rather than trying to predict what git will decide is stash-able, and
what is not, we can just use git to do the autostashing directly. It's
been supported since at least git 2.6 (Sep 2015), which we can assume
has been deployed to enough users to rely on being available.

I do this by adapting the existing 'update existing checkout' code to
include --autostash when it calls git-rebase and removing a bunch of
now-redundant code.

It passes test suite and some test updates I've done locally, including
modules with stash-able changes.