.arcconfig

blackbox: do not cache file objects

Having the blackbox file objects cached in `ui._bbfp` could in theory be
troublesome if multiple processes (ex. chg servers) have file objects
referring to a same file. (Although I spent some time and failed to build a
convincing test case)

This patch makes blackbox re-open the file every time to make the situation
better. Ideally we also need proper locking.

The caching logic traces back to the commit introducing blackbox
(18242716a). That commit does not have details about why caching is
necessary.  Consider the fact that blackbox logs are not many, it seems fine
to remove the fp cache to be more confident.

Differential Revision: https://phab.mercurial-scm.org/D650

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