After doing e.g. hg update -m and all files get automatically
merged, we currently leave the merge state in place. That means that
the previously checked-out commit node gets displayed as % in the
log output. It also means that the user can re-resolve the files using
hg resolve, possibly using a different merge tool. I can see some
minor value in that, although I've never used it myself. However,
commands that commit the changes right away (e.g. rebase, histedit,
graft, backout, evolve) don't let the user re-resolve merged files
without redoing the whole operation. I don't think we've had requests
for that, either, so I don't think there's much reason to think that
it is useful for operations that don't commit. This patch therefore
makes us clear the mergestate if there are no unresolved conflicts.
This has a large impact on the test-merge-changedelete.t test
because that heavily exercised the re-resolve functionality.