import metadata from feeds

When annex.genmetadata is set, metadata from the feed is added to files
that are imported from it.

Reused the same feedtitle and itemtitle, feedauthor, itemauthor, etc names
that are used in --template.

Also added title and author, which are the item title/author if available,
falling back to the feed title/author. These are more likely to be common
metadata fields.

(There is a small bit of dupication here, but once git gets
around to packing the object, it will compress it away.)

The itempubdate field is not included in the metadata as a string; instead
it is used to generate year and month fields, same as is done when adding
files with annex.genmetadata set.

This commit was sponsored by Amitai Schlair, who cooincidentially
is responsible for ikiwiki generating nice feed metadata!

Logs/MetaData.hs

@@ -95,10 +95,12 @@ addMetaData k metadata = addMetaData' k metadata =<< liftIO getPOSIXTime
  - will tend to be generated across the different log files, and so
  - git will be able to pack the data more efficiently. -}
 addMetaData' :: Key -> MetaData -> POSIXTime -> Annex ()
-addMetaData' k (MetaData m) now = Annex.Branch.change (metaDataLogFile k) $
-	showLog . simplifyLog 
-		. S.insert (LogEntry now metadata)
-		. parseLog
+addMetaData' k d@(MetaData m) now
+	| d == emptyMetaData = noop
+	| otherwise = Annex.Branch.change (metaDataLogFile k) $
+		showLog . simplifyLog 
+			. S.insert (LogEntry now metadata)
+			. parseLog
   where
 	metadata = MetaData $ M.filterWithKey (\f _ -> not (isLastChangedField f)) m