When validating whether a PR is mergeable on GitHub, sometimes the PR
will get stuck in an `UNKNOWN` merge state and block merging, even if
it's mergeable in the GitHub UI.

Co-authored-by: Shay Nehmad <[email protected]>

Fixes #55.

Co-authored-by: Shay Nehmad <[email protected]>

Before this change we incorrectly tried to rebase the bottom commit
which resulted in malformed state of the result.

This change adds the `--stash` or `-s` option to the `submit` command
parser. If set, it will run `git stash save` before any commands in the
normal `submit` workflow and, when done, it will run `git stash pop` to
undo this mechanism.

…e template. (#73)

Previously the branch number was always at the end after `/`. This PR
generalizes it and allows it to be present anywhere in the template
under the placeholder "$ID".

Fixes #57.