git-gui: Bind Tab/Shift-Tab to cycle between panes in blame

The blame viewer is composed of two different areas, the file
area on top and the commit area on the bottom.  If users are
trying to shift the focus it is probably because they want to
shift from one area to the other, so we just setup Tab and
Shift-Tab to jump from the one half to the other in a cycle.

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

lib/blame.tcl

@@ -272,6 +272,8 @@ constructor new {i_commit i_path} {
 			set cursorW %W
 			tk_popup $w.ctxm %X %Y
 		"
+		bind $i <Shift-Tab> "[list focus $w_cviewer];break"
+		bind $i <Tab>       "[list focus $w_cviewer];break"
 	}
 
 	foreach i [concat $w_columns $w_cviewer] {
@@ -287,8 +289,10 @@ constructor new {i_commit i_path} {
 		bind $i <Control-Key-f> {catch {%W yview scroll  1 pages};break}
 	}
 
+	bind $w_cviewer <Shift-Tab> "[list focus $w_file];break"
+	bind $w_cviewer <Tab>       "[list focus $w_file];break"
 	bind $w_cviewer <Button-1> [list focus $w_cviewer]
-	bind $top <Visibility> [list focus $top]
+	bind $w_file    <Visibility> [list focus $w_file]
 
 	grid configure $w.header -sticky ew
 	grid configure $w.file_pane -sticky nsew