Fix shell-script-mode indentation of continuation lines

* lisp/progmodes/sh-script.el (sh-smie--default-backward-token):
Fix indentation of continuation lines (bug#44592).

lisp/progmodes/sh-script.el

@@ -1775,7 +1775,7 @@ Does not preserve point."
                      (goto-char p)
                      nil))))
         (while
-            (progn (skip-syntax-backward "w_'")
+            (progn (skip-syntax-backward ".w_'")
                    (or (not (zerop (skip-syntax-backward "\\")))
                        (when (eq ?\\ (char-before (1- (point))))
                          (let ((p (point)))

test/lisp/progmodes/sh-script-tests.el

@@ -0,0 +1,40 @@
+;;; sh-script-tests.el --- Tests for sh-script.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'sh-script)
+(require 'ert)
+
+(ert-deftest test-sh-script-indentation ()
+  (with-temp-buffer
+    (insert "relative-path/to/configure --prefix=$prefix\\
+             --with-x")
+    (shell-script-mode)
+    (goto-char (point-min))
+    (forward-line 1)
+    (indent-for-tab-command)
+    (should (equal
+             (buffer-substring-no-properties (point-min) (point-max))
+             "relative-path/to/configure --prefix=$prefix\\
+			   --with-x"))))
+
+;;; sh-script-tests.el ends here