context: use manifest.find() instead of two separate calls
I noticed this while debugging an extension that's implementing the manifest
interface. Always nice to save a function call.
Differential Revision: https://phab.mercurial-scm.org/D8109
Mercurial
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Mercurial is a fast, easy to use, distributed revision control tool
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$ hg debuginstall # sanity-check setup
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Running without installing::
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