hgrc(5) already implies that this works, so we might as well support it.
Another approach would be to implement this in util.findexe(): that
would benefit other callers of findexe(), e.g. convert and anyone
calling the user's editor. But findexe() is really implemented in
both posix.py and windows.py, so this would make both of those modules
depend on util.py: not good. So keep it narrow and only for merge
tools.
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
