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.
Previously, pull would not update if new branch heads were received,
whereas pull && update would move to the tipmost branch head.
Also change the "crosses branches" abort in merge.update from
"crosses branches (merge branches or use --check to force update)"
to
"crosses branches (merge branches or update --check to force update)"
since it can no longer assume the user is running hg update.
