git-gui: Enable verbose Tcl loading earlier

git-gui: Enable verbose Tcl loading earlier

When we are using our "non-optimized" tclIndex format (which is
just a list of filenames, in the order necessary for source'ing)
we are doing all of our loading before we even tested to see if
GITGUI_VERBOSE was set in the environment.  This meant we never
showed the files as we sourced them into the environment.

Now we setup our overloaded auto_load and source scripts before
we attempt to define our library path, or source the scripts that
it mentions.  This way GITGUI_VERBOSE is always honored if set.

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