Some improvements for support/patch-update:

- Added a --shell option which starts a sub-shell on each patch branch.
- Don't allow the user to exit a sub-shell if the branch is not clean.
- If the sub-shell exited with a non-zero exit status, prompt to see if
  the user wanted to abort rather than assuming that.
- Wait to start the new patch-file output until after the shell runs.
- Always return to the starting branch on exit.