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>