help: clarify distinction among `contains`/`file`/`filelog` For a Mercurial new-comer, the distinction between `contains(x)`, `file(x)`, and `filelog(x)` in the "revsets" help page may not be obvious. This commit tries to make things more obvious (text based on an explanation from Matt in an FB group thread).

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.