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.
#!/bin/sh
hg init
echo a > a
hg add a
hg commit -m "test" -d "0 0"
hg history
hg tag -d "0 0" "bleah"
hg history
echo foo >> .hgtags
hg tag -d "0 0" "bleah2" || echo "failed"