git-gui (MinGW): make use of MSys2's msgfmt

When Git for Windows was still based on MSys1, we had no gettext, ergo no msgfmt, either. Therefore, we introduced a small and simple Tcl script to perform the same task. However, with MSys2, we no longer need that because we have a proper msgfmt executable. Plus, the po2msg.sh script somehow manages to hang when run in parallel in Git for Windows' SDK (symptom: the Continuous Testing tasks timing out). Two reasons to use real msgfmt.exe instead. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>