It's sometimes useful to create a `RepoLoader` given an existing
`ReadonlyRepo`. We already do that in `ReadonlyRepo::reload()`. This
patch repurposes `ReadonlyRepo::reload()` for that.
The only way to load the repo at a current operation (as with
`--at-op`) is currently to first load it at the head operation and
then call `reload()` on the repo. This patch makes it so we can load
the repo directly at the requested operation.
This commits makes it so that running commands outside a repo results
in an error message instead of a panic.
We still don't look for a `.jj/` directory in ancestors of the current
directory.
