revlog: only use the `_indexfp` method for read operation This will avoid "other" code to not overlook `_writing` usage. We introduces private method dedicated to writing to make use the right option are always used. Differential Revision: https://phab.mercurial-scm.org/D10610

revlog: use `_writing` in `rewrite_sidedata` Special cases aren't special enough to break the rules. Differential Revision: https://phab.mercurial-scm.org/D10609

revlog: open files in 'r+' instead of 'a+' The code doing actual writing is already doing the necessary seeking, so we could safely use 'r+'. This make the file objecs usable in more situation, like updating the sidedata information during pull. revlog: forcibly move the file cursor at the right location before writing This is a paranoid change in case the changelog computation moved the cursors under our feets. This is not known to happens right now. Differential Revision: https://phab.mercurial-scm.org/D10608

revlog: pass a transaction object to `rewrite_sidedata` The `_writing` context need one, so we update the function signature before considering using `_writing` in rewrite_sidedata. Differential Revision: https://phab.mercurial-scm.org/D10607