Deduplicate feeds by feed URL, not link element.

Previously, we used the RSS <link> to determine if a feed was a duplicate, but
it's common for multiple feeds to exist for a single site. This changes the
logic to just deduplicate based on the actual link given to us by a user.

I also fixed a bug where we checked more than 1 duplicate instead of just
checking for any duplicates.

See #899 and #883

Author: brendanlong

plugins/backend/decsync/decsyncInterface.vala

@@ -386,7 +386,7 @@ public class FeedReader.decsyncInterface : FeedServerInterface {
 
 		if(feed != null)
 		{
-			if(!db.feed_exists(feed.getURL()))
+			if(!db.feed_exists(feed.getXmlUrl()))
 			{
 				db.write_feeds(ListUtils.single(feed));
 

plugins/backend/local/localInterface.vala

@@ -391,7 +391,7 @@ public class FeedReader.localInterface : FeedServerInterface {
 
 		if(feed != null)
 		{
-			if(!db.feed_exists(feed.getURL()))
+			if(!db.feed_exists(feed.getXmlUrl()))
 			{
 				db.write_feeds(ListUtils.single(feed));
 				return true;

src/DataBaseReadOnly.vala

@@ -677,12 +677,11 @@ public class FeedReader.DataBaseReadOnly : GLib.Object {
 					return id;
 				}
 
-				public bool feed_exists(string feed_url)
+				public bool feed_exists(string xml_url)
 				{
-					var rows = m_db.execute("SELECT COUNT(*) FROM main.feeds WHERE url = ? LIMIT 1", { feed_url });
+					var rows = m_db.execute("SELECT COUNT(*) FROM main.feeds WHERE xmlURL = ? LIMIT 1", { xml_url });
 					assert(rows.size == 1 && rows[0].size == 1);
-					// FIXME: Why > 1 and not > 0?
-					return rows[0][0].to_int() > 1;
+					return rows[0][0].to_int() > 0;
 				}
 
 				public Feed? read_feed(string feedID)