commitctx: document a fast path in _filecommit
This block cut off a lot of logic, documenting the why and how seems useful to
future reader.
This is part of a larger refactoring/cleanup of the commitctx code to clarify
and augment the logic gathering metadata useful for copy tracing. The current
code is a tad too long and entangled to make such update easy. We start with
easy and small cleanup.
Differential Revision: https://phab.mercurial-scm.org/D8700
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