sequencer (rebase -i): update refs after a successful rebase

An interactive rebase operates on a detached HEAD (to keep the reflog
of the original branch relatively clean), and updates the branch only
at the end.

Now that the sequencer learns to perform interactive rebases, it also
needs to learn the trick to update the branch before removing the
directory containing the state of the interactive rebase.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

sequencer.c

@@ -96,6 +96,8 @@ static GIT_PATH_FUNC(stopped_sha, "rebase-merge/stopped-sha")
  * The following files are written by git-rebase just after parsing the
  * command-line (and are only consumed, not modified, by the sequencer).
  */
+static GIT_PATH_FUNC(head_name, "rebase-merge/head-name")
+static GIT_PATH_FUNC(onto, "rebase-merge/onto")
 static GIT_PATH_FUNC(orig_head, "rebase-merge/orig-head")
 static GIT_PATH_FUNC(git_path_rebase_verbose, "rebase-merge/verbose") /* TODO: turn into opt */
 
@@ -1599,12 +1601,39 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 	}
 
 	if (IS_REBASE_I()) {
-		struct strbuf buf = STRBUF_INIT;
+		struct strbuf head_ref = STRBUF_INIT, buf = STRBUF_INIT;
 
 		/* Stopped in the middle, as planned? */
 		if (todo_list->current < todo_list->nr)
 			return 0;
 
+		if (strbuf_read_file(&head_ref, head_name(), 64) > 0 &&
+				starts_with(head_ref.buf, "refs/")) {
+			unsigned char head[20], orig[20];
+
+			strbuf_rtrim(&head_ref);
+			if (get_sha1("HEAD", head))
+				return error("Cannot read HEAD");
+			if (strbuf_read_file(&buf, orig_head(), 41) <= 0 ||
+					get_sha1_hex(buf.buf, orig))
+				return error("Could not read orig-head");
+			strbuf_addf(&buf, "rebase -i (finish): %s onto ",
+				head_ref.buf);
+			strbuf_read_file(&buf, onto(), 64);
+			if (update_ref(buf.buf, head_ref.buf, head, orig,
+					REF_NODEREF, UPDATE_REFS_MSG_ON_ERR))
+				return error("Could not update %s",
+					head_ref.buf);
+			strbuf_reset(&buf);
+			strbuf_addf(&buf,
+				"rebase -i (finish): returning to %s",
+				head_ref.buf);
+			if (create_symref("HEAD", head_ref.buf, buf.buf))
+				return error("Could not update HEAD to %s",
+					head_ref.buf);
+			strbuf_reset(&buf);
+		}
+
 		if (file_exists(git_path_rebase_verbose())) {
 			const char *argv[] = {
 				"diff-tree", "--stat", NULL, NULL
@@ -1619,6 +1648,7 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 			strbuf_reset(&buf);
 		}
 		strbuf_release(&buf);
+		strbuf_release(&head_ref);
 	}
 
 	/*