git-gui: Gracefully handle bad TCL_PATH at compile time

git-gui: Gracefully handle bad TCL_PATH at compile time

Petr Baudis pointed out the main git.git repository's Makefile dies
now if git-gui 0.7.0-rc1 or later is being used and TCL_PATH was not
set to a working tclsh program path.  This breaks people who may have
a working build configuration today and suddenly upgrade to the latest
git release.

The tclIndex is required for git-gui to load its associated lib files,
but using the Tcl auto_load procedure to source only the files we need
is a performance optimization.  We can emulate the auto_load by just
source'ing every file in that directory, assuming we source class.tcl
first to initialize our crude class system.

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