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.
Is there an error message when trying to diff non-existing files?
not: No such file or directory
found: No such file or directory
Is there an error message when trying to add non-existing files?
not: No such file or directory
found: No such file or directory