merge script TODOs

[delta1]

from #923 which was superseded by #1210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• check for required remotes, required branches, etc; notice if merged-master is out of date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