git-gui: Look for gitk in $PATH, not $LIBEXEC/git-core

Signed-off-by: Abhijit Menon-Sen <ams@toroid.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1686,10 +1686,10 @@ proc do_gitk {revs} {
 	# -- Always start gitk through whatever we were loaded with.  This
 	#    lets us bypass using shell process on Windows systems.
 	#
-	set exe [file join [file dirname $::_git] gitk]
+	set exe [_which gitk]
 	set cmd [list [info nameofexecutable] $exe]
-	if {! [file exists $exe]} {
-		error_popup [mc "Unable to start gitk:\n\n%s does not exist" $exe]
+	if {$exe eq {}} {
+		error_popup [mc "Couldn't find gitk in PATH"]
 	} else {
 		global env