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.
1:3aa14bbc23d9
0:8633637036c1
3aa14bbc23d9 tip
8633637036c1
8633637036c1+
8633637036c1
3aa14bbc23d9 tip
merging file1
3aa14bbc23d9 tip
3aa14bbc23d9 tip
3aa14bbc23d9 tip
3aa14bbc23d9 tip