migrate: WORM keys containing spaces will be migrated to not contain spaces anymore

To work around the problem that the external special remote protocol does
not support keys containing spaces.

This commit was sponsored by Denis Dzyubenko on Patreon.

Annex/Locations.hs

@@ -69,6 +69,7 @@ module Annex.Locations (
 	hashDirMixed,
 	hashDirLower,
 	preSanitizeKeyName,
+	reSanitizeKeyName,
 
 	prop_isomorphic_fileKey
 ) where
@@ -426,7 +427,10 @@ gitAnnexAssistantDefaultDir = "annex"
  - same key.
  -}
 preSanitizeKeyName :: String -> String
-preSanitizeKeyName = concatMap escape
+preSanitizeKeyName = preSanitizeKeyName' False
+
+preSanitizeKeyName' :: Bool -> String -> String
+preSanitizeKeyName' resanitize = concatMap escape
   where
 	escape c
 		| isAsciiUpper c || isAsciiLower c || isDigit c = [c]
@@ -435,9 +439,16 @@ preSanitizeKeyName = concatMap escape
 		-- , is safe and uncommon, so will be used to escape
 		-- other characters. By itself, it is escaped to 
 		-- doubled form.
-		| c == ',' = ",,"
+		| c == ',' = if not resanitize
+			then ",,"
+			else ","
 		| otherwise = ',' : show (ord c)
 
+{- Converts a keyName that has been santizied with an old version of
+ - preSanitizeKeyName to be sanitized with the new version. -}
+reSanitizeKeyName :: String -> String
+reSanitizeKeyName = preSanitizeKeyName' True
+
 {- Converts a key into a filename fragment without any directory.
  -
  - Escape "/" in the key name, to keep a flat tree of files and avoid