Only use cp -a if it is supported, falling back to cp -p or plain cp.

* cp --reflink=auto is used if supported, and will make git annex unlock
  much faster on filesystems like btrfs that support copy of write.

CopyFile.hs

@@ -0,0 +1,24 @@
+{- git-annex file copying
+ -
+ - Copyright 2010 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module CopyFile (copyFile) where
+
+import Utility
+import qualified SysConfig
+
+{- The cp command is used, because I hate reinventing the wheel,
+ - and because this allows easy access to features like cp --reflink. -}
+copyFile :: FilePath -> FilePath -> IO Bool
+copyFile src dest = boolSystem "cp" opts
+	where
+		opts = if (SysConfig.cp_reflink_auto)
+			then ["--reflink=auto", src, dest]
+			else if (SysConfig.cp_a)
+				then ["-a", src, dest]
+				else if (SysConfig.cp_p)
+					then ["-p", src, dest]
+					else [src, dest]