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

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>