git-gui: set NO_MSGFMT to force using pure tcl replacement in msysgit

Shawn's git-gui Makefile supports the pure tcl replacement
for msgfmt if setting NO_MSGFMT. This patch sets the NO_MSGFMT
for msysgit.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -126,6 +126,9 @@ ifeq ($(uname_S),Darwin)
 		GITGUI_MACOSXAPP := YesPlease
 	endif
 endif
+ifneq (,$(findstring MINGW,$(uname_S)))
+	NO_MSGFMT=1
+endif
 
 ifdef GITGUI_MACOSXAPP
 GITGUI_MAIN := git-gui.tcl