rebase: guard against missing files in read_basic_state()

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase: finish_rebase() in fast-forward rebase

In the following case

  $ git rebase master
  Fast-forwarded autostash-fix to master.

The autostash is not applied automatically, because this codepath
forgets to call finish_rebase().  Fix this.  Also add a test to guard
against regressions.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase: finish_rebase() in noop rebase

In the following case

  $ git rebase master
  Current branch autostash-fix is up to date.

the autostash is not applied automatically, because this codepath
forgets to call finish_rebase().  Fix this.  Also add a test to guard
against regressions.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>