Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.
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
Without specifying the parent revision of the working copy, users will
update to tip, which is most likely the other head they were trying to
merge, not the revision they were at before the merge.
