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