Fix "File: " skipping when building the elements of the imenu

We shouldn't be using `skip-chars-forward' here as this function skips
over any _character set_ passed as argument, not just a string.

Without this patch, for instance entries like:

  File: lol/foo.bar

Will be added to the candidates list as "ol/foo.bar" as ?l is in the
character set "File: ".

Instead, just skips 6 chars forward.

rg-result.el

@@ -777,7 +777,7 @@ previous file with grouped matches."
               (while (setq nextfile (rg-navigate-file-message nextfile nil 1))
                 (save-excursion
                   (goto-char nextfile)
-                  (skip-chars-forward "File: ")
+                  (and (looking-at-p "^File: ") (forward-char 6))
                   (setq filepath (buffer-substring-no-properties (point) (line-end-position))))
                 (push (cons filepath nextfile) elements))
               (nreverse elements))))))

test/data/limenu.el

@@ -0,0 +1 @@
+imenu

test/rg.el-test.el

@@ -343,6 +343,7 @@ matching alias."
         (should (eq called 'compilation-previous-file))
         (should (eq arg 1))))))
 
+
 (ert-deftest rg-unit/match-grouped-filename ()
   "Test that `rg-match-grouped-filename' finds correct match and restores state."
   (let (saved-pos)
@@ -619,6 +620,18 @@ method. "
                   (rg-project-root "/tmp/foo.el"))
                  "/tmp/")))
 
+(ert-deftest rg-integration-test/imenu-populated ()
+  "Test that the imenu entries are correct."
+  :tags '(need-rg)
+  (rg-run "imenu" "elisp" (concat default-directory "test/data"))
+  (with-current-buffer (rg-buffer-name)
+    (rg-wait-for-search-result)
+    (let ((ientries (funcall imenu-create-index-function)))
+      (should (equal (length ientries) 1))
+      (should (equal
+               '("limenu.el")
+               (seq-sort #'string-lessp (mapcar #'car ientries)))))))
+
 (ert-deftest rg-integration/command-hiding-hide ()
   "Test command hiding when `rg-hide-command` is non nil."
   :tags '(need-rg)