git-gui: Simplify MSGFMT setting in Makefile

git-gui: Simplify MSGFMT setting in Makefile

To prepare msg files for Tcl scripts, the command that is set to MSGFMT
make variable needs to be able to grok "--tcl -l <lang> -d <here>" options
correctly.  This patch simplifies the tests done in git-gui's Makefile to
directly test this condition.  If the test run does not exit properly with
zero status (either because you do not have "msgfmt" itself, or your
"msgfmt" is too old to grok --tcl option --- the reason does not matter),
have it fall back to po/po2msg.sh

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>