filemerge: switch trymerge boolean to mergetype enum
trymerge = False becomes mergetype = nomerge, and trymerge = True becomes
mergetype = fullmerge or mergeonly, depending on whether a premerge happens.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.