sidedata: enable sidedata computers to optionally rewrite flags
Sidedata computers may want to influence the flags of the revision they touch.
For example, the computer for changelog-based copytracing can add a flag to
signify that this revision might affect copytracing, inversely removing said
flag if the information is no longer applicable.
See inline documentation in storageutil for more details.
Differential Revision: https://phab.mercurial-scm.org/D10344