git-gui: Maintain the same file list for diff during refresh.

I just noticed that a file was always jumping to compare against HEAD
and the index during a refresh, even if the diff viewer was comparing
the index against the working directory prior to the refresh.  The
bug turned out to be caused by a foreach loop going through all file
list names searching for the path.  Since $ui_index was the first one
searched and the file was contained in that file list the loop broke
out, leaving $w set to $ui_index when it had been set by the caller
to $ui_workdir.  Silly bug caused by using a parameter as a loop
index.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -607,16 +607,13 @@ proc show_diff {path w {lno {}}} {
 	if {$diff_active || ![lock_index read]} return
 
 	clear_diff
-	if {$w eq {} || $lno == {}} {
-		foreach w [array names file_lists] {
-			set lno [lsearch -sorted $file_lists($w) $path]
-			if {$lno >= 0} {
-				incr lno
-				break
-			}
+	if {$lno == {}} {
+		set lno [lsearch -sorted $file_lists($w) $path]
+		if {$lno >= 0} {
+			incr lno
 		}
 	}
-	if {$w ne {} && $lno >= 1} {
+	if {$lno >= 1} {
 		$w tag add in_diff $lno.0 [expr {$lno + 1}].0
 	}