status: be prepared for not-yet-started interactive rebase

Some developers might want to call `git status` in a working
directory where they just started an interactive rebase, but the
edit script is still opened in the editor.

Let's show a meaningful message in such cases.

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

t/t7512-status-help.sh

@@ -944,4 +944,23 @@ EOF
 	test_i18ncmp expected actual
 '
 
+test_expect_success 'status: handle not-yet-started rebase -i gracefully' '
+	ONTO=$(git rev-parse --short HEAD^) &&
+	COMMIT=$(git rev-parse --short HEAD) &&
+	EDITOR="git status --untracked-files=no >actual" git rebase -i HEAD^ &&
+	cat >expected <<EOF &&
+On branch several_commits
+No commands done.
+Next command to do (1 remaining command):
+   pick $COMMIT four_commit
+  (use "git rebase --edit-todo" to view and edit)
+You are currently editing a commit while rebasing branch '\''several_commits'\'' on '\''$ONTO'\''.
+  (use "git commit --amend" to amend the current commit)
+  (use "git rebase --continue" once you are satisfied with your changes)
+
+nothing to commit (use -u to show untracked files)
+EOF
+	test_i18ncmp expected actual
+'
+
 test_done

wt-status.c

@@ -1072,14 +1072,17 @@ static void abbrev_sha1_in_line(struct strbuf *line)
 	strbuf_list_free(split);
 }
 
-static void read_rebase_todolist(const char *fname, struct string_list *lines)
+static int read_rebase_todolist(const char *fname, struct string_list *lines)
 {
 	struct strbuf line = STRBUF_INIT;
 	FILE *f = fopen(git_path("%s", fname), "r");
 
-	if (!f)
+	if (!f) {
+		if (errno == ENOENT)
+			return -1;
 		die_errno("Could not open file %s for reading",
 			  git_path("%s", fname));
+	}
 	while (!strbuf_getline_lf(&line, f)) {
 		if (line.len && line.buf[0] == comment_line_char)
 			continue;
@@ -1089,6 +1092,7 @@ static void read_rebase_todolist(const char *fname, struct string_list *lines)
 		abbrev_sha1_in_line(&line);
 		string_list_append(lines, line.buf);
 	}
+	return 0;
 }
 
 static void show_rebase_information(struct wt_status *s,
@@ -1103,8 +1107,10 @@ static void show_rebase_information(struct wt_status *s,
 		struct string_list yet_to_do = STRING_LIST_INIT_DUP;
 
 		read_rebase_todolist("rebase-merge/done", &have_done);
-		read_rebase_todolist("rebase-merge/git-rebase-todo", &yet_to_do);
-
+		if (read_rebase_todolist("rebase-merge/git-rebase-todo",
+					 &yet_to_do))
+			status_printf_ln(s, color,
+				_("git-rebase-todo is missing."));
 		if (have_done.nr == 0)
 			status_printf_ln(s, color, _("No commands done."));
 		else {