generate more compact git-annex branch for imports

Especially from borg, where the content identifier logs
all end up being the same identical file!

But also, for other imports, the location tracking logs can,
in some cases, be identical files.

Bonus optimisation: Avoid looking up (and parsing when set)
GIT_ANNEX_VECTOR_CLOCK env var every time a log is written to.
Although the lookup does happen at startup even when no
log will be written now.

Command/MetaData.hs

@@ -74,7 +74,7 @@ optParser desc = MetaDataOptions
 seek :: MetaDataOptions -> CommandSeek
 seek o = case batchOption o of
 	NoBatch -> do
-		c <- liftIO currentVectorClock
+		c <- currentVectorClock
 		let ww = WarnUnmatchLsFiles
 		let seeker = AnnexedFileSeeker
 			{ startAction = start c o
@@ -188,7 +188,7 @@ startBatch (si, (i, (MetaData m))) = case i of
 			, keyOptions = Nothing
 			, batchOption = NoBatch
 			}
-		t <- liftIO currentVectorClock
+		t <- currentVectorClock
 		-- It would be bad if two batch mode changes used exactly
 		-- the same timestamp, since the order of adds and removals
 		-- of the same metadata value would then be indeterminate.