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>

macosx/AppMain.tcl

@@ -7,7 +7,7 @@ if {[string first -psn [lindex $argv 0]] == 0} {
 }
 
 if {[file tail [lindex $argv 0]] eq {gitk}} {
-	set argv0 [file join $gitexecdir gitk]
+	set argv0 [lindex $argv 0]
 	set AppMain_source $argv0
 } else {
 	set argv0 [file join $gitexecdir [file tail [lindex $argv 0]]]