shelve: let cmdutil.revert() take care of backing up untracked files
cmdutil.revert() backs up untracked files, so I don't see a reason to
do it shelve.mergefiles(). We have tests for this and they still pass.
Differential Revision: https://phab.mercurial-scm.org/D6174
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"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
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