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flag if the information is no longer applicable.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
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2