cache remotes via annex-speculate-present

Added remote.name.annex-speculate-present config that can be used to
make cache remotes.

Implemented it in Remote.keyPossibilities, which is used by the
get/move/copy/mirror commands, and nothing else. This way, things like
whereis will not show content that's speculatively present.

The assistant and sync --content were not using Remote.keyPossibilities,
and were changed to use it.

The efficiency hit should be small; Remote.keyPossibilities is only
used before transferring a file, which is the expensive operation.
And, it's only doing one lookup of the remoteList and a very cheap
filter over it.

Note that, git-annex still updates the location log when copying content
to a remote with annex-speculate-present set. In this case, the location
tracking will indicate that content is present in the remote. This may
not be wanted for caches, or may not be a real problem for them. TBD.

This commit was supported by the NSF-funded DataLad project.

Remote.hs

@@ -1,6 +1,6 @@
 {- git-annex remotes
  -
- - Copyright 2011 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -278,13 +278,21 @@ keyLocations key = trustExclude DeadTrusted =<< loggedLocations key
 
 {- Cost ordered lists of remotes that the location log indicates
  - may have a key.
+ -
+ - Also includes remotes with remoteAnnexSpeculatePresent set.
  -}
 keyPossibilities :: Key -> Annex [Remote]
 keyPossibilities key = do
 	u <- getUUID
 	-- uuids of all remotes that are recorded to have the key
 	locations <- filter (/= u) <$> keyLocations key
-	fst <$> remoteLocations locations []
+	speclocations <- map uuid
+		. filter (remoteAnnexSpeculatePresent . gitconfig)
+		<$> remoteList
+	-- there are unlikely to be many speclocations, so building a Set
+	-- is not worth the expense
+	let locations' = speclocations ++ filter (`notElem` speclocations) locations
+	fst <$> remoteLocations locations' []
 
 {- Given a list of locations of a key, and a list of all
  - trusted repositories, generates a cost-ordered list of