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
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