File['/a/common']: symlink to common-local; replace => False

Before If74ea635f, File['/a/common'] was a directory with replace => false
because "# FIXME: /a/common is a symlink on some systems". The system
(singular) was terbium. In fact, it ought to be a symlink everywhere except on
tin, so make it a symlink and set it to replace => false.

Change-Id: If923e3a6c09ac5255a25828dc56ef903460064ad