git-gui: Locate the library directory early during startup

git-gui: Locate the library directory early during startup

To support a localized version of git-gui we need to locate the
library directory early so we can initialize Tcl's msgcat package
to load translated messages from.  This needs to occur before we
declare our git-version proc so that errors related to locating
git or assessing its version can be reported to the end-user in
their preferred language.  However we have to keep the library
loading until after git-version has been declared, otherwise we
will fail to start git-gui if we are using a fake tclIndex that
was generated by our Makefile.

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