backout: use context manager for config override
Differential Revision: https://phab.mercurial-scm.org/D3742
$ hg debugtemplate '{""|splitlines|commondir}\n'
$ hg debugtemplate '{"foo/bar\nfoo/baz\nfoo/foobar\n"|splitlines|commondir}\n'
foo
$ hg debugtemplate '{"foo/bar\nfoo/bar\n"|splitlines|commondir}\n'
foo
$ hg debugtemplate '{"/foo/bar\n/foo/bar\n"|splitlines|commondir}\n'
foo
$ hg debugtemplate '{"/foo\n/foo\n"|splitlines|commondir}\n'
$ hg debugtemplate '{"foo/bar\nbar/baz"|splitlines|commondir}\n'
$ hg debugtemplate '{"foo/bar\nbar/baz\nbar/foo\n"|splitlines|commondir}\n'
$ hg debugtemplate '{"foo/../bar\nfoo/bar"|splitlines|commondir}\n'
foo
$ hg debugtemplate '{"foo\n/foo"|splitlines|commondir}\n'
$ hg init
$ hg log -r null -T '{rev|commondir}'
hg: parse error: argument is not a list of text
(template filter 'commondir' is not compatible with keyword 'rev')
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