sync: Abort rebase in progress if force-checkout is set

This will make "repo sync -d --force-checkout" more reliable
in CI automation, as there are fewer things in the way that may
need manual intervention.

Bug: b/40015382
Change-Id: I8a79971724a3d9a8e2d682b7a0c04deda9e34177
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/423317


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Erik Elmeke <erik@haleytek.corp-partner.google.com>
Commit-Queue: Erik Elmeke <erik@haleytek.corp-partner.google.com>