nice git ack space optimisation when setting the same metadata value for multiple files

Command/MetaData.hs

@@ -14,6 +14,7 @@ import Logs.MetaData
 import Types.MetaData
 
 import qualified Data.Set as S
+import Data.Time.Clock.POSIX
 
 def :: [Command]
 def = [withOptions [setOption] $ command "metadata" paramPaths seek
@@ -30,19 +31,20 @@ setOption = Option ['s'] ["set"] (ReqArg mkmod "field[+-]=value") "set metadata"
 seek :: CommandSeek
 seek ps = do
 	modmeta <- Annex.getState Annex.modmeta
-	withFilesInGit (whenAnnexed $ start modmeta) ps
+	now <- liftIO getPOSIXTime
+	withFilesInGit (whenAnnexed $ start now modmeta) ps
 
-start :: [ModMeta] -> FilePath -> (Key, Backend) -> CommandStart
-start ms file (k, _) = do
+start :: POSIXTime -> [ModMeta] -> FilePath -> (Key, Backend) -> CommandStart
+start now ms file (k, _) = do
 	showStart "metadata" file
-	next $ perform k ms
+	next $ perform now ms k
 
-perform :: Key -> [ModMeta] -> CommandPerform
-perform k [] = next $ cleanup k
-perform k ms = do
+perform :: POSIXTime -> [ModMeta] -> Key -> CommandPerform
+perform _ [] k = next $ cleanup k
+perform now ms k = do
 	oldm <- getCurrentMetaData k
 	let m = foldl' unionMetaData newMetaData $ map (modMeta oldm) ms
-	addMetaData k m
+	addMetaData' k m now
 	next $ cleanup k
 	
 cleanup :: Key -> CommandCleanup

Logs/MetaData.hs

@@ -31,6 +31,7 @@ module Logs.MetaData (
 	setMetaData,
 	unsetMetaData,
 	addMetaData,
+	addMetaData',
 	currentMetaData,
 ) where
 
@@ -70,12 +71,17 @@ setMetaData' isset k field s = addMetaData k $
 {- Adds in some metadata, which can override existing values, or unset
  - them, but otherwise leaves any existing metadata as-is. -}
 addMetaData :: Key -> MetaData -> Annex ()
-addMetaData k metadata = do
-        now <- liftIO getPOSIXTime
-	Annex.Branch.change (metaDataLogFile k) $
-		showLog . simplifyLog 
-			. S.insert (LogEntry now metadata) 
-			. parseLog
+addMetaData k metadata = addMetaData' k metadata =<< liftIO getPOSIXTime
+
+{- Reusing the same timestamp when making changes to the metadata
+ - of multiple keys is a nice optimisation. The same metadata lines
+ - 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 now = Annex.Branch.change (metaDataLogFile k) $
+	showLog . simplifyLog 
+		. S.insert (LogEntry now metadata) 
+		. parseLog
 
 {- Simplify a log, removing historical values that are no longer
  - needed.