git-gui: Skip nicknames when selecting author initials

git-gui: Skip nicknames when selecting author initials

Our blame viewer only grabbed the first initial of the git.git
author string "Simon 'corecode' Schubert".  Here the problem was we
looked at Simon, pulled the S into the author initials, then saw
the single quote as the start of the next name and did not like
this character as it was not an uppercase letter.

We now skip over single quoted nicknames placed within the author
name field and grab the initials following it.  So the above name
will get the initials SS, rather than just S.

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