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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>

Author: jrn