context: get filesadded() and filesremoved() from changeset if configured This adds the read side for getting the sets of added and removed files from the changeset extras. I timed this command on the hg repo: hg log -T '{rev}\n {files}\n %:{file_mods}\n +{file_adds}\n -{file_dels}\n' It took 1m21s before and 6.4s after. I also used that command to check that the result didn't change compared to calculating the values from the manifests on the fly (it didn't change). In the mozilla-unified repo, the same command run on FIREFOX_BETA_58_END::FIREFOX_BETA_59_END went from 29s to 0.67s. Differential Revision: https://phab.mercurial-scm.org/D6417

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