Proper attribution is important in general, but especially for
external contributors. Before this change, and the previous change
fixing authorship deduplication, it was rather easy for a "squashed"
conflict commit (attributed to the 'bot for lack of a really clean
option) to get merged by mistake.
This commit changes two things:
* The mergebot now refuses to stage commits without an email set, the
github API rejects those commits anyway so any integration mode
other than `merge` would fail, just with a very unclear error
* The forwardbot now creates commits with an empty author / committer
email when the pull request as a whole has multiple authors /
committers. This leverages the mergebot update.
Also clean up the staging process to provide richer error reporting
using bespoke exceptions instead of simple assertions. I'm not sure
we've ever encountered most of these errors so they're really sanity
checks but the old reporting would be... less than great.
Fixes#505
1. if we try to stage a PR and realize we'd stored / checked the wrong
head, cancel the staging and notify the PR
2. provide a command to forcefully update pr heads (or at least check
that a PR's head is up to date)
Closes#241
