Preserve metadata when staging a new version of an annexed file.

Performance impact: When adding a large tree of new files, this needs
to do some git cat-file queries to check if any of the files already
existed and might need a metadata copy. I tried a benchmark in a copy
of my sound repository (so there was already a significant git tree
to check against.

Adding 10000 small files, with a cold cache:
  before: 1m48.539s
  after:  1m52.791s

So, impact is 0.0004 seconds per file added. Which seems acceptable, so did
not add some kind of configuration to enable/disable this.

This commit was sponsored by Lisa Feilen.

Annex/MetaData.hs

@@ -11,6 +11,7 @@ import Common.Annex
 import qualified Annex
 import Types.MetaData
 import Logs.MetaData
+import Annex.CatFile
 
 import qualified Data.Set as S
 import qualified Data.Map as M
@@ -27,18 +28,27 @@ yearMetaField = MetaField "year"
 monthMetaField :: MetaField
 monthMetaField = MetaField "month"
 
-{- Generates metadata for a file that has just been ingested into the
- - annex. Passed the FileStatus of the content file.
+{- Adds metadata for a file that has just been ingested into the
+ - annex, but has not yet been committed to git.
  -
- - Does not overwrite any existing metadata values for the key.
+ - When the file has been modified, the metadata is copied over
+ - from the old key to the new key. Note that it looks at the old key as
+ - committed to HEAD -- the new key may or may not have already been staged
+ - in th annex.
+ -
+ - Also, can generate new metadata, if configured to do so.
  -}
-genMetaData :: Key -> FileStatus -> Annex ()
-genMetaData key status = whenM (annexGenMetaData <$> Annex.getGitConfig) $ do
-	metadata <- getCurrentMetaData key
-	let metadata' = genMetaData' status metadata
-	unless (metadata' == emptyMetaData) $
-		addMetaData key metadata'
+genMetaData :: Key -> FilePath -> FileStatus -> Annex ()
+genMetaData key file status = do
+	maybe noop (flip copyMetaData key) =<< catKeyFileHEAD file
+	whenM (annexGenMetaData <$> Annex.getGitConfig) $ do
+		metadata <- getCurrentMetaData key
+		let metadata' = genMetaData' status metadata
+		unless (metadata' == emptyMetaData) $
+			addMetaData key metadata'
 
+{- Generates metadata from the FileStatus.
+ - Does not overwrite any existing metadata values. -}
 genMetaData' :: FileStatus -> MetaData -> MetaData
 genMetaData' status old = MetaData $ M.fromList $ filter isnew
 	[ (yearMetaField, S.singleton $ toMetaValue $ show y)