repoview: add a new filtername for accessing hidden commits
This patch adds a new filter 'visible-hidden' for repository and will be used to
return a repo object with user passed revisions unhidden. Unlike the
directaccess extension in fb-hgext and previous series adding the functionality,
this time we introduce only one new filter as whether to warn user or not is
handled by scmutil.unhidehashlikerevs().
Differential Revision: https://phab.mercurial-scm.org/D1734
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https://www.mercurial-scm.org/wiki/ContributingChanges
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If you can't run the entire testsuite for some reason (it can be
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any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).