Make Build/BuildVersion update embedded version number

This way, autobuilders can run it before building, in a tree where
git-annex was already built, and get the current git rev embedded in the
build.

Before, the version number only updated when the setup program was run,
which was up to cabal and not on every build.

Build/Version.hs

@@ -1,4 +1,4 @@
-{- Package version determination, for configure script. -}
+{- Package version determination. -}
 
 {-# OPTIONS_GHC -fno-warn-tabs #-}
 
@@ -13,7 +13,6 @@ import Prelude
 
 import Utility.Monad
 import Utility.Exception
-import Utility.Directory
 
 type Version = String
 
@@ -33,7 +32,6 @@ getVersion = do
 	ifM (isReleaseBuild)
 		( return changelogversion
 		, catchDefaultIO changelogversion $ do
-			let major = takeWhile (/= '.') changelogversion
 			gitversion <- takeWhile (\c -> isAlphaNum c) <$> readProcess "sh"
 				[ "-c"
 				, "git log -n 1 --format=format:'%h'"
@@ -54,3 +52,18 @@ getChangelogVersion = do
 	return $ middle (words verline !! 1)
   where
 	middle = drop 1 . init
+
+writeVersion :: Version -> IO ()
+writeVersion = writeFile "Build/Version" . body
+  where
+	body ver = unlines $ concat
+		[ header
+		, ["packageversion :: String"]
+		, ["packageversion = \"" ++ ver ++ "\""]
+		, footer
+		]
+	header = [
+		  "{- Automatically generated. -}"
+		, ""
+		]
+	footer = []