[PATCH 08 of 11] filecommit: reuse filenode matching fparents regardless of type(fctx)

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-05-12 00:15:41 +0900:
> Nit: it appears fctx.filenode() is a mandatory API of basefilectx.

Good catch. The only exception is tests/drawdag.py. I'll update it.

> I've double-reviewed up to this patch, and timed out. I'll look the
> remainder tomorrow.

Thanks!