automatic conflict resolution for v6 unlocked files

Several tricky parts:

* When the conflict is just between the same key being locked and unlocked,
  the unlocked version wins, and the file is not renamed in this case.

* Need to update associated file map when conflict resolution renames
  an unlocked file.

* git merge runs the smudge filter on the conflicting file, and actually
  overwrites the file with the same content it had before, and so
  invalidates its inode cache. This makes it difficult to know when it's
  safe to remove such files as conflict cruft, without going so far as to
  compare their entire contents.

  Dealt with this by preventing the smudge filter from populating the file
  when a merge is run. However, that also prevents the smudge filter being
  run for non-conflicting files, so eg moving a file won't put its new
  content into place.

* Ideally, if a merge or a merge conflict resolution renames an unlocked
  file, the file in the work tree can just be moved, rather than copying
  the content to a new worktree file.

  This is attempted to be done in merge conflict resolution, but
  due to git merge's behavior of running smudge filters, what actually
  seems to happen is the old worktree file with the content is deleted and
  rewritten as a pointer file, so doesn't get reused.

So, this is probably not as efficient as it optimally could be.
If that becomes a problem, could look into running the merge in a separate
worktree and updating the real worktree more efficiently, similarly to the
direct mode merge. However, the direct mode merge had a lot of bugs, and
I'd rather not use that more error-prone method unless really needed.

Command/Smudge.hs

@@ -19,6 +19,9 @@ import Utility.InodeCache
 import Types.KeySource
 import Backend
 import Logs.Location
+import Annex.Index (addGitEnv)
+import Utility.Env
+import qualified Git
 import qualified Database.Keys
 
 import qualified Data.ByteString.Lazy as B
@@ -56,7 +59,7 @@ smudge file = do
 			-- don't provide such modified content as it
 			-- will be confusing. inAnnex will detect such
 			-- modifications.
-			ifM (inAnnex k)
+			ifM ((not <$> smudgeDisabled) <&&> inAnnex k)
 				( do
 					content <- calcRepo (gitAnnexLocation k)
 					liftIO $ B.putStr . fromMaybe b
@@ -66,6 +69,16 @@ smudge file = do
 			Database.Keys.addAssociatedFile k file
 	stop
 
+-- Environment variable to disable smudging providing the content of keys.
+smudgeDisabled :: Annex Bool
+smudgeDisabled = liftIO $ isJust <$> getEnv smudgeDisableEnv
+
+smudgeDisableEnv :: String
+smudgeDisableEnv = "ANNEX_SMUDGE_DISABLE"
+
+withSmudgeDisabled :: (Git.Repo -> IO a) -> Annex a
+withSmudgeDisabled a = inRepo $ \r -> addGitEnv r smudgeDisableEnv "1" >>= a
+
 -- Clean filter is fed file content on stdin, decides if a file
 -- should be stored in the annex, and outputs a pointer to its
 -- injested content.