rust: Add a Filelog struct that wraps Revlog
Some filelog-specific logic is moved from code `rhg cat` into this struct
where it can better be reused.
Additionally, a missing end delimiter for metadata causes an error
to be returned instead of being silently ignored.
Differential Revision: https://phab.mercurial-scm.org/D11408
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IncludeCategories:
- Regex: '^<'
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