ui.forcemerge is set before calling into merge or resolve commands, then unset
to prevent ui pollution for further operations.
ui.forcemerge takes precedence over HGMERGE, but mimics HGMERGE behavior if the
given --tool is not found by the merge-tools machinery. This makes it possible
to do: hg resolve --tool="python mymerge.py" FILE
With this approach, HGMERGE and ui.merge are not harmed by --tool
