Markdown reader: fix parsing of "bare locators"...

...after author-in-text citations.

Previously `@item [p. 12; @item2]` was incorrectly parsed as
three citations rather than two.  This is now fixed by ensuring
that `prefix` doesn't gobble any semicolons.

src/Text/Pandoc/Readers/Markdown.hs

@@ -2216,7 +2216,7 @@ suffix = try $ do
 
 prefix :: PandocMonad m => MarkdownParser m (F Inlines)
 prefix = trimInlinesF . mconcat <$>
-  manyTill inline (char ']'
+  manyTill (notFollowedBy (char ';') >> inline) (char ']'
    <|> lookAhead
          (try $ do optional (try (char ';' >> spnl))
                    citeKey True