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
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