Merge procedure can lead to mistakes on master

[rptb1]

@thejayps accidentally made commits with incorrect messages on master by following proc.merge.pull-request because he committed after resolving conflicts, not realising that would commit to master.

We can make three improvements:

1. We could encourage the use of catch-up merges from master to resolve conflict.
2. We can warn people not to commit when resolving conflicts from the merge until they are tested, since this changes master.
3. We could tell people to eyeball the log of master before pushing, e.g. with git log -n 3 --first-parent master