gitk: Fix "External diff" with separate work tree

Running "External diff" to compare the index and work tree currently
brings up an empty blame view when the work tree is not the parent of
the git directory.  This is because the file that is taken from the
work tree is assumed to be in
$GIT_DIR/../<repo-relative-file-name>.  Fix it by feeding the diff tool
a path under $GIT_WORK_TREE instead of "$GIT_DIR/..".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -3364,10 +3364,10 @@ proc save_file_from_commit {filename output what} {
 
 proc external_diff_get_one_file {diffid filename diffdir} {
     global nullid nullid2 nullfile
-    global gitdir
+    global worktree
 
     if {$diffid == $nullid} {
-        set difffile [file join [file dirname $gitdir] $filename]
+        set difffile [file join $worktree $filename]
 	if {[file exists $difffile]} {
 	    return $difffile
 	}
@@ -11650,6 +11650,7 @@ set cdup {}
 if {$isworktree} {
     set cdup [exec git rev-parse --show-cdup]
 }
+set worktree [exec git rev-parse --show-toplevel]
 setcoords
 makewindow
 catch {