rebase -i: Teach "--edit-todo" action

This allows users to edit the todo file while they're stopped in the
middle of an interactive rebase. When this action is executed, all
comments from the original todo file are stripped, and new help messages
are appended to the end.

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -792,6 +792,23 @@ skip)
 
 	do_rest
 	;;
+edit-todo)
+	sed -e '/^#/d' < "$todo" > "$todo".new
+	mv -f "$todo".new "$todo"
+	append_todo_help
+	cat >> "$todo" << EOF
+#
+# You are editing the todo file of an ongoing interactive rebase.
+# To continue rebase after editing, run:
+#     git rebase --continue
+#
+EOF
+
+	git_sequence_editor "$todo" ||
+		die "Could not execute editor"
+
+	exit
+	;;
 esac
 
 git var GIT_COMMITTER_IDENT >/dev/null ||