Pull requests can no longer be imported when the branch is deleted

[cjolowicz]

❯ retrocookie-pr 233
⨯ #233 Bump py from 1.9.0 to 1.10.0
⨯ The command git rev-list failed with status 128.

❯ git rev-list --no-walk master..py-1.10.0

fatal: ambiguous argument 'master..py-1.10.0': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'