grep: extend functionality, add man page entry, add unit test. walkchangerevs now returns a two-tuple. Its behaviour is also extensively commented. The annotate command's getname function has been factored out to a new function, trimname, so it can be shared between annotate and grep. The behaviour of grep has been beefed up, so that it now performs a number of useful functions.

precommit hook
commit hook: acb14030fe0a21b60322c440ad2d20cf7685a376