gitk: Fix "blame parent commit" with separate work tree

Running "blame parent commit" currently brings up an empty blame view
when the the work tree is not the parent of the git directory.  Fix it
by feeding git-blame paths relative to $GIT_WORK_TREE instead of
"$GIT_DIR/..".
Signed-off-by: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -3557,7 +3557,7 @@ proc make_relative {f} {
 }
 
 proc external_blame {parent_idx {line {}}} {
-    global flist_menu_file gitdir
+    global flist_menu_file cdup
     global nullid nullid2
     global parentlist selectedline currentid
 
@@ -3576,7 +3576,7 @@ proc external_blame {parent_idx {line {}}} {
     if {$line ne {} && $line > 1} {
 	lappend cmdline "--line=$line"
     }
-    set f [file join [file dirname $gitdir] $flist_menu_file]
+    set f [file join $cdup $flist_menu_file]
     # Unfortunately it seems git gui blame doesn't like
     # being given an absolute path...
     set f [make_relative $f]