test: update all specs for Store instance API

Problem: every spec used the old singleton API (store.unload(),
store.load(), store.add(), etc.) and diff.apply(lines, hidden).

Solution: lower-level specs (store, diff, views, complete, file) use
s = store.new(path); s:load() directly. Higher-level specs (archive,
edit, filter, status, sync) reset package.loaded['pending'] in
before_each and use pending.store() to access the live instance.
diff.apply calls updated to diff.apply(lines, s, hidden_ids).

spec/complete_spec.lua

@@ -3,6 +3,7 @@ require('spec.helpers')
 local buffer = require('pending.buffer')
 local config = require('pending.config')
 local store = require('pending.store')
+local buffer = require('pending.buffer')
 
 describe('complete', function()
   local tmpdir