git-gui: Correctly handle UTF-8 encoded commit messages

git-gui: Correctly handle UTF-8 encoded commit messages

Uwe Kleine-König discovered git-gui mangled his surname and did
not send the proper UTF-8 byte sequence to git-commit-tree when
his name appeared in the commit message (e.g. Signed-Off-By line).

Turns out this was related to other trouble that I had in the past
with trying to use "fconfigure $fd -encoding $enc" to select the
stream encoding and let Tcl's IO engine do all of the encoding work
for us.  Other parts of git-gui were just always setting the file
channels to "-encoding binary" and then performing the encoding
work themselves using "encoding convertfrom" and "convertto", as
that was the only way I could make UTF-8 filenames work properly.

I found this same bug in the amend code path, and in the blame
display.  So its fixed in all three locations (commit creation,
reloading message for amend, viewing  message in blame).

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