gitk: Fix "show origin of this line" with separate work tree

Running "show origin of this line" currently fails 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

@@ -3589,7 +3589,7 @@ proc external_blame {parent_idx {line {}}} {
 proc show_line_source {} {
     global cmitmode currentid parents curview blamestuff blameinst
     global diff_menu_line diff_menu_filebase flist_menu_file
-    global nullid nullid2 gitdir
+    global nullid nullid2 gitdir cdup
 
     set from_index {}
     if {$cmitmode eq "tree"} {
@@ -3642,7 +3642,7 @@ proc show_line_source {} {
     } else {
 	lappend blameargs $id
     }
-    lappend blameargs -- [file join [file dirname $gitdir] $flist_menu_file]
+    lappend blameargs -- [file join $cdup $flist_menu_file]
     if {[catch {
 	set f [open $blameargs r]
     } err]} {