[Fix #1212] Cache project-root in non-file-backed buffers

`projectile-cached-buffer-file-name` would be set to nil in non-file-backed
buffers causing the root caching to not work. `'none` is used as a sentinel
value for `projectile-project-root-cache` so this reuses that for
`projectile-cached-buffer-file-name` as well.

CHANGELOG.md

@@ -17,6 +17,7 @@
 
 ### Changes
 
+* [#1213](https://github.com/bbatsov/projectile/pull/1213): Cache project root in non-filed-backed buffers.
 * [#1175](https://github.com/bbatsov/projectile/pull/1175): `projectile-register-project-type` can now set a default compilation directory for build systems that needs to build out-of-tree (eg. meson).
 * [#1175](https://github.com/bbatsov/projectile/pull/1175): `projectile-{test,run}-project` now run inside `(projectile-compilation-dir)`, just like `projectile-compile-project`.
 * [#1175](https://github.com/bbatsov/projectile/pull/1175): `projectile-{test,run}-project` now stores the default command per directory instead of per project, just like `projectile-compile-project`.

projectile.el

@@ -905,10 +905,10 @@ will return the current directory, otherwise it'd raise an error."
   ;; instead
   (or (cl-subst nil 'none
                 (or (and projectile-cached-buffer-file-name
-                         (equal projectile-cached-buffer-file-name buffer-file-name)
+                         (equal projectile-cached-buffer-file-name (or buffer-file-name 'none))
                          projectile-cached-project-root)
                     (progn
-                      (setq projectile-cached-buffer-file-name buffer-file-name)
+                      (setq projectile-cached-buffer-file-name (or buffer-file-name 'none))
                       (setq projectile-cached-project-root
                             ;; The `is-local' and `is-connected' variables are
                             ;; used to fix the behavior where Emacs hangs