git-gui: Initialize Tcl's msgcat library for internationalization

git-gui: Initialize Tcl's msgcat library for internationalization

Tcl's msgcat library and corresponding mc procedure can locate a
translated string for any user message, provided that it is first
given a directory where the *.msg files are located containing the
translations.

During installation we will place the translations in lib/msgs/,
so we need to inform msgcat of this location once we determine it
during startup.  Our source code tree however will store all of
the translations within the po/ directory, so we need to special
case this variant.

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