git-gui:handle the encoding of Git's output correctly

git-gui:handle the encoding of Git's output correctly
if we use 'eval exec $opt $cmdp $args' to execute git command,
tcl engine will convert the output of the git comand with the rule
system default code page to unicode.
But cp936 -> unicode conversion implicitly done by exec is not reversible.
So we have to use git_read instead.

Bug report and an original reproducer by Cloud Chou:
https://github.com/msysgit/git/issues/302

Karsten Blees writes this code patch.
Cloud Chou find the reason of the bug.

Thanks-to: dscho
Thanks-to: patthoyts
Signed-off-by: Karsten Blees <blees@dcon.de>
Original-test-by: Cloud Chou <515312382@qq.com>
Signed-off-by: Cloud Chou <515312382@qq.com>