Make it possible to oot-rebase the master branch

Otherwise it fails, refusing to update the current branch in the temporary repo
oot-rebase: General goodness

Use “env” to look up bash.  Use -e to catch errors instead of chained &&s.
Add -o pipefail in case evolution ever introduces a pipe.
oot-rebase: "general goodness" fixes

- Give only `bash` to /usr/bin/env, to please `env`s that doesn't split words
- Make `shift` not exit the script if it fails