Remove the --force flag to "stg rebase" and "stg pull"

Instead, always behave as if the force flag was given; that is, don't
check if rebasing would leave a dangling commit behind. Reasons:

  * The check for this was very strict and caused a lot of false
    positives.

  * Everything is recorded in the reflog, so we can't actually lose
    commits.

This fixes bug 9181.

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