discovery: don't report all "unsynced" remote heads (issue4230)
8a9e0b523d2d made discovery more helpful - too helpful for some extreme use
cases.
Instead, we arbitrarily limit the list it at 4 and add 'or more'.
rebase: don't abort if we're asked to rebase an empty revset
The documentation says we exit 1 if we have nothing to do, so avoid
breaking that contract when we're passed an empty revset.
This was changed in http://www.selenic.com/hg/rev/a259f7b488ab to
improve the error message; keep the improved message, just not the
abort.