This means that the commits previously pinned by remote branches are no longer
abandoned. I think that's more correct since "push" is the operation to
propagate local view to remote, and uninteresting commits should have been
locally abandoned.
The original idea was to completely replace git_refs with remotes["git"] by
introducing "forgotten" state, but it turned out to break "fetch && undo"
scenario. There are other ways around, but they also have problems:
* Sets tombstone on forgotten/deleted remote refs, exports remote refs without
comparing to the known refs.
* `jj undo` would need to insert tombstone by diffing old/new views.
* `jj branch forget` would need to preserve the @git branch whereas the other
remote branches would be forgotten.
* Always overwrites remote refs on export.
* `jj git export` without importing would discard remote refs.
So, I decided to not remove git_refs. Apparently, it also improves the undo
behavior. In the new model, `jj git fetch && jj undo && jj git fetch` works
even if git_refs isn't rolled back. So we can unify the default of
`jj undo --what`.
