speed up very first tree import by 25%

Reading from the cidsdb is responsible for about 25% of the runtime of
an import. Since the cidmap is used to store the same information in
ram, the cidsdb is not written to during an import any longer. And so,
if it started off empty (and updateFromLog wasn't needed), those reads
can just be skipped.

This is kind of a cheesy optimisation, since after any import from any
special remote, the database will no longer be empty, so it's a single
use optimisation. But it's probably not uncommon to start by importing a
lot of files, and it can save a lot of time then.

Sponsored-by: Brock Spratlen on Patreon

Remote/Helper/ExportImport.hs

@@ -280,14 +280,14 @@ adjustExportImport' isexport isimport r rs = do
 			Nothing -> ifM (liftIO $ atomically $ tryPutTMVar lcklckv ())
 				( do
 					db <- ContentIdentifier.openDb
-					ContentIdentifier.needsUpdateFromLog db >>= \case
+					db' <- ContentIdentifier.needsUpdateFromLog db >>= \case
 						Just v -> do
 							cidlck <- calcRepo' gitAnnexContentIdentifierLock 
 							withExclusiveLock cidlck $
 								ContentIdentifier.updateFromLog db v
-						Nothing -> noop
-					liftIO $ atomically $ putTMVar dbtv db
-					return db
+						Nothing -> pure db
+					liftIO $ atomically $ putTMVar dbtv db'
+					return db'
 				-- loser waits for winner to open the db and
 				-- can then also use its handle
 				, liftIO $ atomically (readTMVar dbtv)