git: add readfast() method to manifest
Mysteriously, on a different system a handful of things don't work
without this, including log. This is especially odd to be because log
works in the test. In any event, it's legal for readfast() to just
return read(), so that's what we do.
Differential Revision: https://phab.mercurial-scm.org/D8262
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"base": "hg:.^"
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