fix relPathDirToFileAbs on windows with different drive letters

Since we started using this for git repos, when a remote was on another
drive, it resulted in a bogus relative path to it being used by git-annex,
which didn't work.

Utility/Path.hs

@@ -138,9 +138,15 @@ relPathDirToFile from to = relPathDirToFileAbs <$> absPath from <*> absPath to
 
 {- This requires the first path to be absolute, and the
  - second path cannot contain ../ or ./
+ -
+ - On Windows, if the paths are on different drives,
+ - a relative path is not possible and the path is simply
+ - returned as-is.
  -}
 relPathDirToFileAbs :: FilePath -> FilePath -> FilePath
-relPathDirToFileAbs from to = join s $ dotdots ++ uncommon
+relPathDirToFileAbs from to
+	| takeDrive from /= takeDrive to = to
+	| otherwise = join s $ dotdots ++ uncommon
   where
 	s = [pathSeparator]
 	pfrom = split s from

debian/changelog

@@ -10,6 +10,8 @@ git-annex (5.20150410) UNRELEASED; urgency=medium
   * Windows: Renamed start menu file to avoid loop in some versions
     of Windows where the menu file is treated as a git-annex program.
   * bittorrent: Fix handling of magnet links.
+  * Windows: Fixed support of remotes on other drives.
+    (A reversion introduced in version 5.20150113.)
 
  -- Joey Hess <id@joeyh.name>  Thu, 09 Apr 2015 20:59:43 -0400
 

doc/bugs/Windows:_Annex_can_not_get_files.mdwn

@@ -158,3 +158,5 @@ ok
 C:\annex1>cd \annex2
 
 """]]
+
+> [[fixed|done]]; a simple path calculation bug. --[[Joey]]

doc/bugs/Windows:_repo_located_on_different_drive_letter_unavailable.mdwn

@@ -160,3 +160,5 @@ Latest sync command should inject annex-uuid to .config file, but it does not. F
     [remote "c"]
         url = C:\\Annex
         fetch = +refs/heads/*:refs/remotes/c/*
+
+> [[fixed|done]]; a simple path calculation bug. --[[Joey]]