revlog: always open revlogs for reading and appending An upcoming patch will teach revlogs to use the existing file handle to read revision data instead of opening a new file handle just for quick reads. For this to work, files must be opened for reading as well. This patch is merely cosmetic: there are no behavior changes.

revlog: support using an existing file handle when reading revlogs Currently, the low-level revlog reading code always opens a new file handle. In some key scenarios, the revlog is already opened and an existing file handle could be used to read. This patch paves the road to that by teaching various revlog reading functions to accept an optional existing file handle to read from.