revpair: drop useless conditional

The `if not revs:` case is tested at the beginning of the function.
repo: rephrase the "missing requirement" error message

Unknown requirements will now be reported as:
  abort: repository requires features unknown to this Mercurial: largefiles!
  (see http://mercurial.selenic.com/wiki/MissingRequirement for more information)

Some features of this phrasing:
* avoid double ':' in abort message
* make it more clear who requires and knows what
* don't quote the requirement names - it is not something the user entered or
  need the exact spelling of ... and it is "identifiers" that are unambiguous
  anyway
* remove double hint by removing "(upgrade Mercurial)" comment
* don't mention upgrading Mercurial without mentioning enabling the feature -
  instead, just refer to wiki page for both
* don't just talk about "details", talk about "more information"
tests: add run-tests --changed option for running tests changed in revisions

Convenient when polishing patches and changing details of how they change test
output.

This will probably break in weird ways for revsets with special quoting ... but
it is good enough for run-tests.

Usage example:
  yes | ./run-tests.py -li --changed qparent