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/SingleValue.hs

@@ -0,0 +1,65 @@
+{- git-annex single-value log
+ -
+ - This is used to store a value in a way that can be union merged.
+ -
+ - A line of the log will look like: "timestamp value"
+ -
+ - The line with the newest timestamp wins.
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.SingleValue where
+
+import Common.Annex
+import qualified Annex.Branch
+
+import qualified Data.Set as S
+import Data.Time.Clock.POSIX
+import Data.Time
+import System.Locale
+
+class Serializable v where
+	serialize :: v -> String
+	deserialize :: String -> Maybe v
+
+data LogEntry v = LogEntry
+	{ changed :: POSIXTime
+	, value :: v
+	} deriving (Eq, Show, Ord)
+
+type Log v = S.Set (LogEntry v)
+
+showLog :: (Serializable v) => Log v -> String
+showLog = unlines . map showline . S.toList
+  where
+	showline (LogEntry t v) = unwords [show t, serialize v]
+
+parseLog :: (Ord v, Serializable v) => String -> Log v
+parseLog = S.fromList . mapMaybe parse . lines
+  where
+	parse line = do
+		let (ts, s) = splitword line
+		date <- utcTimeToPOSIXSeconds <$> parseTime defaultTimeLocale "%s%Qs" ts
+		v <- deserialize s
+		Just (LogEntry date v)
+	splitword = separate (== ' ')
+
+newestValue :: Log v -> Maybe v
+newestValue s
+	| S.null s = Nothing
+	| otherwise = Just (value $ S.findMax s)
+
+readLog :: (Ord v, Serializable v) => FilePath -> Annex (Log v)
+readLog = parseLog <$$> Annex.Branch.get
+
+getLog :: (Ord v, Serializable v) => FilePath -> Annex (Maybe v)
+getLog = newestValue <$$> readLog
+
+setLog :: (Serializable v) => FilePath -> v -> Annex ()
+setLog f v = do
+        now <- liftIO getPOSIXTime
+        let ent = LogEntry now v
+	Annex.Branch.change f $ \_old -> showLog (S.singleton ent)