git-gui: Fix commit encoding handling.

git-gui: Fix commit encoding handling.

Commits without an encoding header are supposed to
be encoded in utf8. While this apparently hasn't always
been the case, currently it is the active convention, so
it is better to follow it; otherwise people who have to
use commitEncoding on their machines are unable to read
utf-8 commits made by others.

I also think that it is preferrable to display the warning
about an unsupported value of commitEncoding more prominently,
because this condition may lead to surprising behavior and,
eventually, to loss of data.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>