git-gui: Fallback to Tcl based po2msg.sh if msgfmt isn't available

If msgfmt fails with exit code 127 that typically means the program is not found in the user's PATH and thus cannot be executed by make. In such a case we can try to fallback to the Tcl based po2msg program that we distributed with git-gui, as it does a "good enough" job. We still don't default to po2msg.sh however as it does not perform a lot of the sanity checks that msgfmt does, and quite a few of those are too useful to give up. Signed-off-by: Shawn O. Pearce <spearce@spearce.org>