bundlerepo: build revlog index with flags
This fixes bundlerevlog.flags(rev) for any revisions provided by the bundle.
Now test-flagprocessor.t points us to another issue.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).