use GIT keys for export of non-annexed files

This solves the problem that import of such files gets confused and
converts them back to annexed files.

The import code already used GIT keys internally when it determined a
file should not be annexed. So now when it sees a GIT key that export
used, it already does the right thing.

This also means that even older version of git-annex can import and will
do the right thing, once a fixed version has exported. Still, there may
be other complications around upgrades; still need to think it all
through.

Moved gitShaKey and keyGitSha from Key to Annex.Export since they're
only used for export/import.

Documented GIT keys in backends, since they do appear in the git-annex
branch now.

This commit was sponsored by Graham Spencer on Patreon.

Key.hs

@@ -23,8 +23,6 @@ module Key (
 	nonChunkKey,
 	chunkKeyOffset,
 	isChunkKey,
-	gitShaKey,
-	keyGitSha,
 	isKeyPrefix,
 	splitKeyNameExtension,
 
@@ -37,7 +35,6 @@ import qualified Data.Attoparsec.ByteString as A
 
 import Common
 import Types.Key
-import Git.Types
 import Utility.QuickCheck
 import Utility.Bloom
 import Utility.Aeson
@@ -61,23 +58,6 @@ chunkKeyOffset k = (*)
 isChunkKey :: Key -> Bool
 isChunkKey k = isJust (fromKey keyChunkSize k) && isJust (fromKey keyChunkNum k)
 
--- Encodes a git sha as a key.
---
--- This is not the same as a SHA1 key, because the mapping needs to be
--- bijective, also because git may not always use SHA1.
-gitShaKey :: Sha -> Key
-gitShaKey (Ref s) = mkKey $ \kd -> kd
-	{ keyName = s
-	, keyVariety = OtherKey "GIT"
-	}
-
--- Reverse of gitShaKey
-keyGitSha :: Key -> Maybe Sha
-keyGitSha k
-	| fromKey keyVariety k == OtherKey "GIT" =
-		Just (Ref (fromKey keyName k))
-	| otherwise = Nothing
-
 serializeKey :: Key -> String
 serializeKey = decodeBS' . serializeKey'