git-gui: chmod +x po2msg, windows/git-gui.sh

The Makefile only runs po/po2msg.sh using tclsh, but because the
script has the usual tcl preamble starting with #!/bin/sh it can also
be run directly.

The Windows git-gui wrapper is usable in-place for the same reason.

Signed-off-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Pat Thoyts <[email protected]>

Author: jrn