sequencer: add advice for revert

In the case of merge conflicts, while performing a revert, we are
currently advised to use `git cherry-pick --<sequencer-options>`.
Introduce a separate advice message for `git revert`. Also change
the signature of `create_seq_dir` to handle which advice to display
selectively.

Signed-off-by: Rohit Ashiwal <rohit.ashiwal265@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/advice.txt

@@ -57,6 +57,8 @@ advice.*::
 	resolveConflict::
 		Advice shown by various commands when conflicts
 		prevent the operation from being performed.
+	sequencerInUse::
+		Advice shown when a sequencer command is already in progress.
 	implicitIdentity::
 		Advice on how to set your identity configuration when
 		your information is guessed from the system username and

advice.c

@@ -15,6 +15,7 @@ int advice_status_u_option = 1;
 int advice_commit_before_merge = 1;
 int advice_reset_quiet_warning = 1;
 int advice_resolve_conflict = 1;
+int advice_sequencer_in_use = 1;
 int advice_implicit_identity = 1;
 int advice_detached_head = 1;
 int advice_set_upstream_failure = 1;
@@ -71,6 +72,7 @@ static struct {
 	{ "commitBeforeMerge", &advice_commit_before_merge },
 	{ "resetQuiet", &advice_reset_quiet_warning },
 	{ "resolveConflict", &advice_resolve_conflict },
+	{ "sequencerInUse", &advice_sequencer_in_use },
 	{ "implicitIdentity", &advice_implicit_identity },
 	{ "detachedHead", &advice_detached_head },
 	{ "setupStreamFailure", &advice_set_upstream_failure },

advice.h

@@ -15,6 +15,7 @@ extern int advice_status_u_option;
 extern int advice_commit_before_merge;
 extern int advice_reset_quiet_warning;
 extern int advice_resolve_conflict;
+extern int advice_sequencer_in_use;
 extern int advice_implicit_identity;
 extern int advice_detached_head;
 extern int advice_set_upstream_failure;

sequencer.c

@@ -2650,15 +2650,38 @@ static int walk_revs_populate_todo(struct todo_list *todo_list,
 	return 0;
 }
 
-static int create_seq_dir(void)
+static int create_seq_dir(struct repository *r)
 {
-	if (file_exists(git_path_seq_dir())) {
-		error(_("a cherry-pick or revert is already in progress"));
-		advise(_("try \"git cherry-pick (--continue | --quit | --abort)\""));
+	enum replay_action action;
+	const char *in_progress_error = NULL;
+	const char *in_progress_advice = NULL;
+
+	if (!sequencer_get_last_command(r, &action)) {
+		switch (action) {
+		case REPLAY_REVERT:
+			in_progress_error = _("revert is already in progress");
+			in_progress_advice =
+			_("try \"git revert (--continue | --abort | --quit)\"");
+			break;
+		case REPLAY_PICK:
+			in_progress_error = _("cherry-pick is already in progress");
+			in_progress_advice =
+			_("try \"git cherry-pick (--continue | --abort | --quit)\"");
+			break;
+		default:
+			BUG("unexpected action in create_seq_dir");
+		}
+	}
+	if (in_progress_error) {
+		error("%s", in_progress_error);
+		if (advice_sequencer_in_use)
+			advise("%s", in_progress_advice);
 		return -1;
-	} else if (mkdir(git_path_seq_dir(), 0777) < 0)
+	}
+	if (mkdir(git_path_seq_dir(), 0777) < 0)
 		return error_errno(_("could not create sequencer directory '%s'"),
 				   git_path_seq_dir());
+
 	return 0;
 }
 
@@ -4242,7 +4265,7 @@ int sequencer_pick_revisions(struct repository *r,
 	 */
 
 	if (walk_revs_populate_todo(&todo_list, opts) ||
-			create_seq_dir() < 0)
+			create_seq_dir(r) < 0)
 		return -1;
 	if (get_oid("HEAD", &oid) && (opts->action == REPLAY_REVERT))
 		return error(_("can't revert as initial commit"));