destutil: provide hint on rebase+merge for how to specify destination/rev
Without a destination specified, rebase and merge attempt to identify a good
candidate; this fails if there's too many heads, the heads have a bookmark, or
other reasons.
Complicating the issue, users may have specified -t, thinking it means target,
and are confused when the error message says that they need to specify an
explicit rev or explicit destination - they feel like they already have.
Differential Revision: https://phab.mercurial-scm.org/D7024
[tool.black]
line-length = 80
exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'