repoview: pin revisions for `local` and `other` when a merge is active

I've hit this a couple of times, where pulling with a dirty `wdir` obsoletes
`p1` and updating to the successor results in merge conflicts.  The problem was
resolving them failed immediately, complaining that the old checkout was
filtered.

The change in `test-rebase-obsolete.t` is because there's an outstanding merge
conflict in a rebase operation.  The summary prompt to merge seems incorrect for
this scenario, but that's an existing issue.

Differential Revision: https://phab.mercurial-scm.org/D8980