When a workspace's working-copy commit is updated from another
workspace, the workspace becomes "stale". That means that the working
copy on disk doesn't represent the commit that the repo's view says it
should. In this state, we currently automatically it to the desired
commit next time the user runs any command in the workspace. That can
be undesirable e.g. if the user had a slow build or test run started
in the working copy. It can also be surprising that a checkout happens
when the user ran a seemingly readonly command like `jj status`.
This patch makes most commands instead error out if the working copy
is stale, and adds a `jj workspace update-stale` to update it. The
user can still run commands with `--no-commit-working-copy` in this
state (doing e.g. `jj --no-commit-working-copy rebase -r @ -d @--` is
another way of getting into the stale-working-copy state, by the way).
We have had support for workspaces for six months, but I forgot to
update the documentation.
This just adds some basic documentation; we can add more later.
