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