difftool: put the cursor on the editable file for Vim

You only need to edit worktree files when comparing against
the worktree.  Put the cursor automatically into its window for
vimdiff and gvimdiff to avoid doing <C-w>l every time.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/difftool/git-difftool-helper

@@ -78,12 +78,16 @@ launch_merge_tool () {
 		"$merge_tool_path" -o "$MERGED" "$LOCAL" "$REMOTE"
 		;;
 
-	meld|vimdiff)
+	meld)
 		"$merge_tool_path" "$LOCAL" "$REMOTE"
 		;;
 
+	vimdiff)
+		"$merge_tool_path" -c "wincmd l" "$LOCAL" "$REMOTE"
+		;;
+
 	gvimdiff)
-		"$merge_tool_path" -f "$LOCAL" "$REMOTE"
+		"$merge_tool_path" -c "wincmd l" -f "$LOCAL" "$REMOTE"
 		;;
 
 	xxdiff)