memfilectx: make changectx argument mandatory in constructor (API)
committablefilectx has three subclasses: workingfilectx, memfilectx,
and overlayfilectx. committablefilectx takes an optional (change) ctx
instance to its constructor. If it's provided, it's set on the
instance as self._changectx. If not, that property is supposed to be
defined by the class. However, only workingfilectx does that. The
other two will have the property undefined if it's not passed in the
constructor. That seems bad to me. This patch makes the changectx
argument to the memfilectx constructor mandatory because that fixes
the failure I ran into. It seems like we should also fix the
overlayfilectx case.
Differential Revision: https://phab.mercurial-scm.org/D1658
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