git-gui: Show author initials in blame groups

git-gui: Show author initials in blame groups

Frequently when I'm looking at blocks of code in the blame
viewer I want to know who is the culprit, or who I should
be praising for a job well done.  The tooltips nicely show
this if I mouse over a block, but it doesn't work to get
this detail at a glance.

Since we don't use the leftmost commit column for anything
after the first line within a commit group I'm now tossing
the author's initials into that field, right justified.  It
is quite clearly not a SHA-1 number as we always show the
SHA-1 in lowercase, while we explicitly select only the
uppercase characters from an author's name field, and only
those that are following whitespace.

I'm using initials here over anything else as they are quite
commonly unique within small development teams.  The leading
part of the email address field was out for some of the teams
I work with, as there the email addresses are all of the form
"Givenname.Surname@initech.com".  That will never fit into the
4 characters available.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>