git-gui: Correct 'Visualize Branches' on Mac OS X to start gitk

git-gui: Correct 'Visualize Branches' on Mac OS X to start gitk

In Git 1.6 and later gitk is in $prefix/bin while git-gui and all
of the other commands are in $gitexecdir, which is typically not
the same as $prefix/bin.  So we cannot launch $gitexecdir/gitk and
expect it to actually start gitk properly.

By allowing git-gui to locate the script via $PATH and then using
exactly that path when we source it during the application start
we can correctly run gitk on any Git 1.5 or later.

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