merge script TODOs

from #923 which was superseded by #1210

evaluate what still needs to be done to improve the script

TODO

- [ ] check whether we're running from /tmp and don't output the "bail out" warning
- [ ] check for chronic and any other tools we might need
- [ ] use ccache rather than skipping autogen/configure (and configure in multiple ways?)
- [ ] don't require running from the project root
- [ ] check for required remotes, required branches, etc; notice if merged-master is out of date
- [ ] notice if there are uncommitted changes and refuse to run
- [ ] add "dry run" mode
- [ ] add mode to validate others' commits, including noticing skipped PRs