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. Git-Mediawiki/Git-Mediawiki@e41ee9b
2. Git-Mediawiki/Git-Mediawiki#61
3. Git-Mediawiki/Git-Mediawiki#29

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: simon04

contrib/mw-to-git/git-remote-mediawiki.perl

@@ -807,7 +807,10 @@ sub get_more_refs {
 sub mw_import {
 	# multiple import commands can follow each other.
 	my @refs = (shift, get_more_refs('import'));
+	my $processedRefs;
 	foreach my $ref (@refs) {
+		next if $processedRefs->{$ref}; # skip duplicates: "import refs/heads/master" being issued twice; TODO: why?
+		$processedRefs->{$ref} = 1;
 		mw_import_ref($ref);
 	}
 	print {*STDOUT} "done\n";