git-gui: Quiet the msgfmt part of the make process

git-gui: Quiet the msgfmt part of the make process

I really prefer having a very short and sweet makefile output that
does not flood the user's screen with a ton of commands that they
don't care much about.  Traditionally git-gui has hidden away the
actual commands from output by the $(QUIET*) series of macros but
allow them to be seen with either `make QUIET=` or `make V=1`.

This change makes our i18n message generation process to be a lot
shorter and easier to digest at a glance:

  GITGUI_VERSION = 0.8.2.19.gb868-dirty
    * new locations or Tcl/Tk interpreter
    GEN git-gui
    BUILTIN git-citool
    INDEX lib/
    MSGFMT    po/de.msg 268 translated.
    MSGFMT    po/hu.msg 268 translated.
    MSGFMT    po/it.msg 268 translated.
    MSGFMT    po/ja.msg 268 translated.
    MSGFMT    po/ru.msg 249 translated, 12 fuzzy, 4 untranslated.
    MSGFMT po/zh_cn.msg 60 translated, 37 fuzzy, 168 untranslated.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>