revlog: use single file handle when de-inlining revlog _getsegmentforrevs() will eventually call into _datareadfp() to resolve a file handle to read revision data. If no file handle is passed into _getsegmentforrevs(), it opens a new one. Explicit is better than implicit. This commit changes _enforceinlinesize() to open a file handle explicitly when converting inline revlogs to split revlogs and to pass this file handle into _getsegmentforrevs(). I haven't measured, but this change should improve performance, as we no longer reopen the revlog for reading for every revision in the revlog when it is converted from inline to split. Instead, we open it at most once and use it for the duration of the operation. That being said, I /think/ the chunk cache may mitigate the number of file opens required. Differential Revision: https://phab.mercurial-scm.org/D5265