Can now be built with older git versions (before 1.7.7); the resulting binary should only be used with old git.

Remove git old version check from configure, and use the git version
it was built against in the git check-attr code.

Git/Version.hs

@@ -0,0 +1,38 @@
+{- git version checking
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.Version where
+
+import Common
+import qualified Build.SysConfig
+
+{- Using the version it was configured for avoids running git to check its
+ - version, at the cost that upgrading git won't be noticed.
+ - This is only acceptable because it's rare that git's version influences
+ - code's behavior. -}
+version :: String
+version = Build.SysConfig.gitversion
+
+older :: String -> Bool
+older v = normalize version < normalize v
+
+{- To compare dotted versions like 1.7.7 and 1.8, they are normalized to
+ - a somewhat arbitrary integer representation. -}
+normalize :: String -> Integer
+normalize = sum . mult 1 . reverse .
+		extend precision . take precision .
+		map readi . split "."
+	where
+		extend n l = l ++ replicate (n - length l) 0
+		mult _ [] = []
+		mult n (x:xs) = (n*x) : mult (n*10^width) xs
+		readi :: String -> Integer
+		readi s = case reads s of
+			((x,_):_) -> x
+			_ -> 0
+		precision = 10 -- number of segments of the version to compare
+		width = length "yyyymmddhhmmss" -- maximum width of a segment