When passing a revision to rbt diff or rbt post in a Jujutsu

repository, a typo in the change ID would produce an unhelpful error:
CRITICAL: Unexpected error executing the command: jj log -r ...

The actual error from jj was swallowed. A revset that matched no

changes, such as none(), crashed with "list index out of range".
parse_revision_spec now raises InvalidRevisionSpecError for these

cases. The message includes the error from jj, such as:
CRITICAL: Invalid revision "kkkkqqqq": Revision `kkkkqqqq` doesn't exist

This also adds special handling for the "zzzzzzzz" root commit, which

would attempt to look up the parent and then fail later with a confusing

fork point error.

Ran unit tests.
Ran rbt diff with a typoed change ID, an empty revset, a revset

  with a syntax error, the root commit ID, and a valid change.