misc/emacs: add support for ff-find-other-file

c-mode classically uses ff-find-other-file to toggle between headers
and implementation. For Go it seemingly makes sense to jump between
implementation and test.

While there's no enforced mapping of file names for tests, the mapping
in this CL seems to be very common at least throughout the standard
library, and ff-find-other-file fails gracefully when the mapping
doesn't apply.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/65750044

misc/emacs/go-mode.el

@@ -7,6 +7,7 @@
 (require 'cl)
 (require 'etags)
 (require 'ffap)
+(require 'find-file)
 (require 'ring)
 (require 'url)
 
@@ -168,6 +169,13 @@ from https://github.com/bradfitz/goimports."
   :type 'string
   :group 'go)
 
+(defcustom go-other-file-alist
+  '(("_test\\.go\\'" (".go"))
+    ("\\.go\\'" ("_test.go")))
+  "See the documentation of `ff-other-file-alist' for details."
+  :type '(repeat (list regexp (choice (repeat string) function)))
+  :group 'go)
+
 (defface go-coverage-untracked
   '((t (:foreground "#505050")))
   "Coverage color of untracked code."
@@ -561,6 +569,8 @@ recommended that you look at goflymake
   (set (make-local-variable 'go-dangling-cache) (make-hash-table :test 'eql))
   (add-hook 'before-change-functions (lambda (x y) (setq go-dangling-cache (make-hash-table :test 'eql))) t t)
 
+  ;; ff-find-other-file
+  (setq ff-other-file-alist 'go-other-file-alist)
 
   (setq imenu-generic-expression
         '(("type" "^type *\\([^ \t\n\r\f]*\\)" 1)