sequencer (rebase -i): write the 'done' file

In the interactive rebase, commands that were successfully processed are
not simply discarded, but appended to the 'done' file instead. This is
used e.g. to display the current state to the user in the output of
`git status` or the progress.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -40,6 +40,12 @@ static GIT_PATH_FUNC(rebase_path, "rebase-merge")
  * file and written to the tail of 'done'.
  */
 static GIT_PATH_FUNC(rebase_path_todo, "rebase-merge/git-rebase-todo")
+/*
+ * The rebase command lines that have already been processed. A line
+ * is moved here when it is first handled, before any associated user
+ * actions.
+ */
+static GIT_PATH_FUNC(rebase_path_done, "rebase-merge/done")
 /*
  * A script to set the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
  * GIT_AUTHOR_DATE that will be used for the commit that is currently
@@ -1296,6 +1302,23 @@ static int save_todo(struct todo_list *todo_list, struct replay_opts *opts)
 		return error_errno(_("could not write to '%s'"), todo_path);
 	if (commit_lock_file(&todo_lock) < 0)
 		return error(_("failed to finalize '%s'."), todo_path);
+
+	if (is_rebase_i(opts)) {
+		const char *done_path = rebase_path_done();
+		int fd = open(done_path, O_CREAT | O_WRONLY | O_APPEND, 0666);
+		int prev_offset = !next ? 0 :
+			todo_list->items[next - 1].offset_in_buf;
+
+		if (fd >= 0 && offset > prev_offset &&
+		    write_in_full(fd, todo_list->buf.buf + prev_offset,
+				  offset - prev_offset) < 0) {
+			close(fd);
+			return error_errno(_("could not write to '%s'"),
+					   done_path);
+		}
+		if (fd >= 0)
+			close(fd);
+	}
 	return 0;
 }