gitk: chmod +x po2msg.sh

The Makefile only runs it using tclsh, but because the fallback po2msg script has the usual tcl preamble starting with #!/bin/sh it can also be run directly. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Paul Mackerras <paulus@samba.org>