setup bug-reference for emacs

* .dir-locals.el: Here.
* tests/core/remfin.test, tests/core/unambig.test:
Make consistent references to issue numbers.
* HACKING: Document convention for mentioning issues.

HACKING

@@ -389,6 +389,16 @@ Comments
   * Prefer C++-style comments (// foo) to C-style comments (/* foo */).
     Use /// for Doxygen comments.
 
+  * When mentioning an issue number, in comments as well as in commit
+    messages, use one of the following forms so that we can configure
+    our editors to turn these into hyperlinks.
+
+       Issue #123 / issue #123 / Fixes #123 / fixes #123
+
+    (When gitlab sees a commit message containing "Fixes #123" or
+    "fixes #123" pushed to branch "next", it will automatically close
+    the issue.)
+
 Formating
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