check onlyActionOn in Drop

* drop -J: Avoid processing the same key twice at the same time when
  multiple annexes files use it.

This prevents a drop of a key conflicting with another drop of the same
key.

This commit was sponsored by Brock Spratlen on Patreon.

CHANGELOG

@@ -16,6 +16,8 @@ git-annex (7.20181106) UNRELEASED; urgency=medium
     to an exporttree remote that already contains the files.
   * smudge: When passed a file located outside the working tree, eg by git
     diff, avoid erroring out.
+  * drop -J: Avoid processing the same key twice at the same time when
+    multiple annexes files use it.
 
  -- Joey Hess <id@joeyh.name>  Tue, 06 Nov 2018 12:44:27 -0400
 

Command/Drop.hs

@@ -68,7 +68,7 @@ start o file key = start' o key afile (mkActionItem afile)
 	afile = AssociatedFile (Just file)
 
 start' :: DropOptions -> Key -> AssociatedFile -> ActionItem -> CommandStart
-start' o key afile ai = do
+start' o key afile ai = onlyActionOn key $ do
 	from <- maybe (pure Nothing) (Just <$$> getParsed) (dropFrom o)
 	checkDropAuto (autoMode o) from afile key $ \numcopies ->
 		stopUnless (want from) $