Group commits per author and add a cover message.

Group the list of detected commits after each poll after author. Before
printing messages for an author print a hardcoded cover message
like "Foobar committed 3 commits to origin/master at My Repo". Output
for each line is unaffected but can typicaly contain less info since
author, repo etc is given by the cover message.
Modify group header to reflect snarfing.

Modify the group header to say "Talking about fa1afe1?" when presenting
commit data for a snarfed hash.
Add a %S -> ' ' substitution

Only meaningful at beginning of line to support indentation, leading
blanks are otherwise hard to express in the .ini file.