add DebugLocks build flag

Using the method described in
https://www.fpcomplete.com/blog/2018/05/pinpointing-deadlocks-in-haskell
but my own code to implement it, and with callstacks added.

This work is supported by the NIH-funded NICEMAN (ReproNim TR&D3) project.

Logs/Transfer.hs

@@ -95,7 +95,7 @@ startTransferInfo afile = TransferInfo
  - interrupted.
  -}
 checkTransfer :: Transfer -> Annex (Maybe TransferInfo)
-checkTransfer t = do
+checkTransfer t = debugLocks $ do
 	tfile <- fromRepo $ transferFile t
 	let lck = transferLockFile tfile
 	let cleanstale = do