merge: implement --tool arguments using new ui.forcemerge configurable 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