merge: make 'keep' message more descriptive
Most merge action messages don't describe the action itself, they
describe the reason the action was taken. The only exeption is the 'k'
action, for which the message is just "keep" and instead there is a
code comment folling it that says "remote unchanged". Let's move that
comment into the merge action message.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.