global numcopies setting

* numcopies: New command, sets global numcopies value that is seen by all
  clones of a repository.
* The annex.numcopies git config setting is deprecated. Once the numcopies
  command is used to set the global number of copies, any annex.numcopies
  git configs will be ignored.
* assistant: Make the prefs page set the global numcopies.

This global numcopies setting is needed to let preferred content
expressions operate on numcopies.

It's also convenient, because typically if you want git-annex to preserve N
copies of files in a repo, you want it to do that no matter which repo it's
running in. Making it global avoids needing to warn the user about gotchas
involving inconsistent annex.numcopies settings.
(See changes to doc/numcopies.mdwn.)

Added a new variety of git-annex branch log file, that holds only 1 value.
Will probably be useful for other stuff later.

This commit was sponsored by Nicolas Pouillard.

Logs.hs

@@ -11,7 +11,11 @@ import Common.Annex
 import Types.Key
 
 {- There are several varieties of log file formats. -}
-data LogVariety = UUIDBasedLog | NewUUIDBasedLog | PresenceLog Key
+data LogVariety
+	= UUIDBasedLog
+	| NewUUIDBasedLog
+	| PresenceLog Key
+	| SingleValueLog
 	deriving (Show)
 
 {- Converts a path from the git-annex branch into one of the varieties
@@ -20,6 +24,7 @@ getLogVariety :: FilePath -> Maybe LogVariety
 getLogVariety f
 	| f `elem` topLevelUUIDBasedLogs = Just UUIDBasedLog
 	| isRemoteStateLog f = Just NewUUIDBasedLog
+	| f == numcopiesLog = Just SingleValueLog
 	| otherwise = PresenceLog <$> firstJust (presenceLogs f)
 
 {- All the uuid-based logs stored in the top of the git-annex branch. -}
@@ -43,6 +48,9 @@ presenceLogs f =
 uuidLog :: FilePath
 uuidLog = "uuid.log"
 
+numcopiesLog :: FilePath
+numcopiesLog = "numcopies.log"
+
 remoteLog :: FilePath
 remoteLog = "remote.log"
 
@@ -118,6 +126,7 @@ prop_logs_sane dummykey = all id
 	, expect isPresenceLog (getLogVariety $ locationLogFile dummykey)
 	, expect isPresenceLog (getLogVariety $ urlLogFile dummykey)
 	, expect isNewUUIDBasedLog (getLogVariety $ remoteStateLogFile dummykey)
+	, expect isSingleValueLog (getLogVariety $ numcopiesLog)
 	]
   where
   	expect = maybe False
@@ -127,3 +136,5 @@ prop_logs_sane dummykey = all id
 	isNewUUIDBasedLog _ = False
 	isPresenceLog (PresenceLog k) = k == dummykey
 	isPresenceLog _ = False
+	isSingleValueLog SingleValueLog = True
+	isSingleValueLog _ = False