git-gui: Assume `blame --incremental` output is in UTF-8

git-gui: Assume `blame --incremental` output is in UTF-8

Most commits have author name encoded in UTF-8, but the incremental
blame output dumps raw bytes and doesn't give us the encoding header
from the commit.  Rather than fixing up tooltip data after we have
viewed that particular commit in the blame viewer we can assume all
names are in UTF-8.

This is still going to cause problems when the author name is not
encoded in UTF-8, but the only (efficient) way to solve that is to
add an "encoding" header to the blame --incremental mode output,
as otherwise we need to run `git cat-file commit $sha1` for each
and every commit identified and that would be horribly expensive
on any platform.

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