don't copy old date metadata when adding new version of a file

When adding a new version of a file, and annex.genmetadata is enabled,
don't copy the data metadata from the old version of the file, instead use
the mtime of the file. Rationalle being that the user has requested to
generate metadata and so would expect to get the new mtime into metadata.

Also, avoid warning about copying metadata when all the old metadata is
date metadata. Which was rather the harder part.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Types/MetaData.hs

@@ -252,13 +252,15 @@ data ModMeta
 	= AddMeta MetaField MetaValue
 	| DelMeta MetaField (Maybe MetaValue)
 	-- ^ delete value of a field. With Just, only that specific value
-	-- is deleted; with Nothing, all current values are deleted.a
+	-- is deleted; with Nothing, all current values are deleted.
 	| DelAllMeta
 	-- ^ delete all currently set metadata
 	| SetMeta MetaField (S.Set MetaValue)
 	-- ^ removes any existing values
 	| MaybeSetMeta MetaField MetaValue
 	-- ^ set when field has no existing value
+	| ComposeModMeta ModMeta ModMeta
+	-- ^ composing multiple modifications
 	deriving (Show)
 
 {- Applies a ModMeta, generating the new MetaData.
@@ -279,6 +281,7 @@ modMeta m (SetMeta f s) = updateMetaData' f s $
 modMeta m (MaybeSetMeta f v)
 	| S.null (currentMetaDataValues f m) = updateMetaData f v emptyMetaData
 	| otherwise = emptyMetaData
+modMeta m (ComposeModMeta a b) = unionMetaData (modMeta m a) (modMeta m b)
 
 {- Avoid putting too many fields in the map; extremely large maps make
  - the seriaization test slow due to the sheer amount of data.