memfilectx: override copysource() instead of using dummy nodeid
The "_copied" property in basefilectx is used by renamed() and
copysource(). committablefilectx (which memfilectx subclasses)
overrides renamed() and writes it in terms of copysource() instead of
_copied. That means that the nodeid part of "_copied" is memfilectx is
unused. Let's instead override copysource() too so we don't need the
"_copied".
Differential Revision: https://phab.mercurial-scm.org/D6159
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