This patch adds a new mergecommit command which is very useful for workflows
where there is automated merging involved. This patch uses existing in-memory
merge API to merge in-memory and create a commit.
I was thinking about --commit flag to merge command, but then we need other
commitopts too, so created a new command.
This patches filemerge.py to not raise IMMConflictsError on first conflict
because we need a list of conflicts in the end.