We used to call forceClone() to update with upstream, but this deletes the
checked out directory. This is inefficient, can delete existing plan files,
and is very surprising if you are working manually in the working directory.

We now fetch an updated upstream, and re-do the merge operation. This
leaves any working files intact.

It's never safe to clone again. But sometimes we need to check
for upstream changes to avoid reverting changes.

The flag is now used to know when we need to merge
again non-destructively with new changes.

As long as the branch itself has not been updated, plans
should be kept. Even if upstream has changed.

This flag was only set to true in case a call to Clone()
ended up merging with an updated upstream, so the
new name better represents what it means.

This complements the test that Clone with unmodified
branch but modified upstream does _not_ wipe plans.

This is no longer a warning, but expected behavior in merge chekout mode

Every call to wrappedGit for the same PR uses identical setup
for directory, head repo and PR, so passing the