repoview: only pin obsolete wdir parents while there are unresolved conflicts
I noticed after doing an update from an obsolete revision with a dirty wdir that
the obsolete commit stayed visible for no obvious reason. It was decided in
85b03b1e4715 not to clear mergestate once all of the conflicts were resolved, in
order to allow re-resolving. Since the point of pinning the obsolete parents
was to allow resolving in the first place (aaeccdb6e654), it makes sense to also
gate it on whether or not there are any remaining files to resolve. This might
result in pinning again if files are marked unresolved again, but that seems
reasonable, given that it still solves the original issue.
Note that this isn't purely cosmetic- pushing with a pinned obsolete revision is
likely to cause complaints about pushing multiple heads or other unexpected
errors. So the faster it comes out of that state, the better.
Differential Revision: https://phab.mercurial-scm.org/D9248