remote-mediawiki: fix duplicate revisions being imported

remote-mediawiki: fix duplicate revisions being imported

Fix a bug with revisions being imported twice. This commit is being
backported from Git-Mediawiki.git's e41ee9b ("All revisions imported
twice", 2018-02-02) to git.git. See [1] for the original commit and
[2] and [3] for the upstream PR and issue.

1. https://github.com/Git-Mediawiki/Git-Mediawiki/commit/e41ee9b3a32416df381cdc79f63350665c84151e
2. https://github.com/Git-Mediawiki/Git-Mediawiki/pull/61
3. https://github.com/Git-Mediawiki/Git-Mediawiki/issues/29

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>