Markdown reader: improved handling of mmd link attributes in references.

Previously they only worked for links that had titles.  Closes #7080.

src/Text/Pandoc/Readers/Markdown.hs

@@ -334,6 +334,8 @@ referenceKey = try $ do
                     skipMany spaceChar
                     notFollowedBy' referenceTitle
                     notFollowedBy' $ guardEnabled Ext_link_attributes >> attributes
+                    notFollowedBy' $ guardEnabled Ext_mmd_link_attributes >>
+                                     try (spnl <* keyValAttr)
                     notFollowedBy' (() <$ reference)
                     many1Char $ notFollowedBy space >> litChar
   let betweenAngles = try $ char '<' >> manyTillChar litChar (char '>')

test/command/7080.md

@@ -0,0 +1,8 @@
+```
+% pandoc -f markdown_mmd -t native
+![][image]
+
+[image]: image.png width=100px height=150px
+^D
+[Para [Image ("",[],[("width","100px"),("height","150px")]) [] ("image.png","")]]
+```