Test "stg rebase" after "stg commit"

Two new tests for "stg rebase":

  1. Try to rebase to a commit that is ahead of HEAD. This should
     work, and does.

  2. Try to commit a patch, and then rebase. This doesn't work,
     because "stg rebase" aborts if orig-base != base, and "stg
     commit" doesn't update orig-base. (It does work if "stg rebase"
     is given the --force flag.)

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Karl Hasselström <kha@treskal.com>