D1243: overlayworkingctx: add `_compact()`

[durin42 (Augie Fackler)]

durin42 added inline comments.

INLINE COMMENTS

> context.py:2242
> +        """Removes keys from the cache that are actually clean, by comparing
> +        them with the underlting context.
> +

typo: s/underlting/underlying/

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D1243

To: phillco, #hg-reviewers
Cc: durin42, mercurial-devel