Update packages/yasnippet by subtree-merging from its external upstream

Again, packages/yasnippet/snippets is left untouched in GNU ELPA. The
upstream version uses a submodule pointing to a snippets repo which
lives at https://github.com/AndreaCrotti/yasnippet-snippets.git

Conflicts: packages/yasnippet/snippets

diff --cc packages/yasnippet/CONTRIBUTING.md
Simple merge

diff --cc packages/yasnippet/README.mdown
Simple merge

diff --cc packages/yasnippet/doc/doc/snippet-expansion.org
Simple merge

diff --cc packages/yasnippet/doc/doc/snippet-menu.org
Simple merge

diff --cc packages/yasnippet/doc/doc/snippet-organization.org
Simple merge

diff --cc packages/yasnippet/doc/yas-doc-helper.el
Simple merge

diff --cc packages/yasnippet/yasnippet-debug.el
Simple merge

diff --cc packages/yasnippet/yasnippet-tests.el
index 2d7a0bc84eb2f7820e12b5e60386ece381b2757f,29a61dccaa950f8ec06fbabaafa24d15ab822b94..648e4f39b2e8b708499d10dd334057e588979461
--- 1/packages/yasnippet/yasnippet-tests.el
--- 2/yasnippet-tests.el
+++ b/packages/yasnippet/yasnippet-tests.el
@@@ -336,12 -335,9 +345,12 @@@ TODO: correct this bug!
                       "brother from another mother") ;; no newline should be here!
              )))
  
 +(defvar yas--barbaz)
 +(defvar yas--foobarbaz)
 +
  ;; See issue #497. To understand this test, follow the example of the
  ;; `yas-key-syntaxes' docstring.
- ;; 
+ ;;
  (ert-deftest complicated-yas-key-syntaxes ()
    (with-temp-buffer
      (yas-saving-variables
@@@ -488,13 -506,41 +519,42 @@@
                            emacs-lisp-mode
                            lisp-interaction-mode))
                (observed (yas--modes-to-activate)))
-          (should (null (cl-set-exclusive-or expected observed)))
-          (should (= (length expected)
-                     (length observed))))))))
+          (should (equal major-mode (car observed)))
+          (should (equal (sort expected #'string<) (sort observed #'string<))))))))
+ 
+ (ert-deftest extra-modes-parenthood ()
+   "Test activation of parents of `yas--extra-modes'."
+   (yas-saving-variables
+    (yas-with-snippet-dirs '((".emacs.d/snippets"
+                              ("c-mode"
+                               (".yas-parents" . "cc-mode"))
+                              ("yet-another-c-mode"
+                               (".yas-parents" . "c-mode and-also-this-one lisp-interaction-mode"))))
+      (yas-reload-all)
+      (with-temp-buffer
+        (yas-activate-extra-mode 'c-mode)
+        (yas-activate-extra-mode 'yet-another-c-mode)
+        (yas-activate-extra-mode 'and-that-one)
+        (let* ((expected-first `(and-that-one
+                                 yet-another-c-mode
+                                 c-mode
+                                 ,major-mode))
+               (expected-rest `(cc-mode
+                                ;; prog-mode doesn't exist in emacs 24.3
+                                ,@(if (fboundp 'prog-mode)
+                                      '(prog-mode))
+                                emacs-lisp-mode
+                                and-also-this-one
+                                lisp-interaction-mode))
+               (observed (yas--modes-to-activate)))
+          (should (equal expected-first
+                         (cl-subseq observed 0 (length expected-first))))
+          (should (equal (sort expected-rest #'string<)
+                         (sort (cl-subseq observed (length expected-first)) #'string<))))))))
  
 +(defalias 'yas--phony-c-mode 'c-mode)
 +
  (ert-deftest issue-492-and-494 ()
 -  (defalias 'yas--phony-c-mode 'c-mode)
    (define-derived-mode yas--test-mode yas--phony-c-mode "Just a test mode")
    (yas-with-snippet-dirs '((".emacs.d/snippets"
                              ("yas--test-mode")))

diff --cc packages/yasnippet/yasnippet.el
Simple merge