sequencer: always discard index after checkout

As the checkout runs in a separate process our index will be out of date
so it should be discarded. The existing callers are not doing this
consistently so do it here to avoid the callers having to worry about
it.

This fixes some test failures that happen if do_interactive_rebase() is
called without forking rebase--interactive which we will implement
shortly. Running

  git rebase -i master topic

starting on master created empty todo lists because all the commits in
topic were marked as cherry-picks. After topic was checked out in
prepare_branch_to_be_rebased() the working tree contained the contents
from topic but the index contained master and the cache entries were
still valid. This meant that diff_populate_filespec() which loads the
blobs when calculating patch-id's ended up reading the contents for
master from the working tree which actually contained topic.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase--interactive.c

@@ -171,7 +171,7 @@ static int do_interactive_rebase(struct replay_opts *opts, unsigned flags,
 	struct argv_array make_script_args = ARGV_ARRAY_INIT;
 	struct todo_list todo_list = TODO_LIST_INIT;
 
-	if (prepare_branch_to_be_rebased(opts, switch_to))
+	if (prepare_branch_to_be_rebased(the_repository, opts, switch_to))
 		return -1;
 
 	if (get_revision_ranges(upstream, onto, &head_hash,

sequencer.c

@@ -3418,10 +3418,11 @@ static const char *reflog_message(struct replay_opts *opts,
 	return buf.buf;
 }
 
-static int run_git_checkout(struct replay_opts *opts, const char *commit,
-			    const char *action)
+static int run_git_checkout(struct repository *r, struct replay_opts *opts,
+			    const char *commit, const char *action)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
+	int ret;
 
 	cmd.git_cmd = 1;
 
@@ -3430,25 +3431,31 @@ static int run_git_checkout(struct replay_opts *opts, const char *commit,
 	argv_array_pushf(&cmd.env_array, GIT_REFLOG_ACTION "=%s", action);
 
 	if (opts->verbose)
-		return run_command(&cmd);
+		ret = run_command(&cmd);
 	else
-		return run_command_silent_on_success(&cmd);
+		ret = run_command_silent_on_success(&cmd);
+
+	if (!ret)
+		discard_index(r->index);
+
+	return ret;
 }
 
-int prepare_branch_to_be_rebased(struct replay_opts *opts, const char *commit)
+int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts,
+				 const char *commit)
 {
 	const char *action;
 
 	if (commit && *commit) {
 		action = reflog_message(opts, "start", "checkout %s", commit);
-		if (run_git_checkout(opts, commit, action))
+		if (run_git_checkout(r, opts, commit, action))
 			return error(_("could not checkout %s"), commit);
 	}
 
 	return 0;
 }
 
-static int checkout_onto(struct replay_opts *opts,
+static int checkout_onto(struct repository *r, struct replay_opts *opts,
 			 const char *onto_name, const char *onto,
 			 const char *orig_head)
 {
@@ -3458,7 +3465,7 @@ static int checkout_onto(struct replay_opts *opts,
 	if (get_oid(orig_head, &oid))
 		return error(_("%s: not a valid OID"), orig_head);
 
-	if (run_git_checkout(opts, onto, action)) {
+	if (run_git_checkout(r, opts, onto, action)) {
 		apply_autostash(opts);
 		sequencer_remove_state(opts);
 		return error(_("could not detach HEAD"));
@@ -4786,7 +4793,7 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 	if (todo_list_parse_insn_buffer(r, new_todo.buf.buf, &new_todo) ||
 	    todo_list_check(todo_list, &new_todo)) {
 		fprintf(stderr, _(edit_todo_list_advice));
-		checkout_onto(opts, onto_name, onto, orig_head);
+		checkout_onto(r, opts, onto_name, onto, orig_head);
 		todo_list_release(&new_todo);
 
 		return -1;
@@ -4805,7 +4812,7 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 
 	todo_list_release(&new_todo);
 
-	if (checkout_onto(opts, onto_name, oid_to_hex(&oid), orig_head))
+	if (checkout_onto(r, opts, onto_name, oid_to_hex(&oid), orig_head))
 		return -1;
 
 	if (require_clean_work_tree(r, "rebase", "", 1, 1))

sequencer.h

@@ -175,7 +175,8 @@ void commit_post_rewrite(struct repository *r,
 			 const struct commit *current_head,
 			 const struct object_id *new_head);
 
-int prepare_branch_to_be_rebased(struct replay_opts *opts, const char *commit);
+int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts,
+				 const char *commit);
 
 #define SUMMARY_INITIAL_COMMIT   (1 << 0)
 #define SUMMARY_SHOW_AUTHOR_DATE (1 << 1)