fix gitAnnexLink to not be absolute on Windows

Windows: Fix reversion that caused the path used to link to annexed
content include the drive letter and full path, rather than being
relative. (`git annex fix` will fix up after this problem).

I've not identified the commit that brought the reversion (probably it
happened this spring when I was removing MisingH and last touched
Utility.Path). Likely commit 18b9a4b8024115db67ae309fdaf54e1553037529?

The problem is that relPathDirToFile got called two paths that had the
slashes different ways around. Since takeDrive includes the first slash,
this made two paths on the same drive seem different and it bailed.

(ifdefs around this to avoid doing extra work on non-windows)

This commit was sponsored by Jack Hill on Patreon.

CHANGELOG

@@ -9,6 +9,9 @@ git-annex (6.20171019) UNRELEASED; urgency=medium
     Thanks, Sean T Parsons 
   * Add day to metadata when annex.genmetadata is enabled.
     Thanks, Sean T Parsons
+  * Windows: Fix reversion that caused the path used to link
+    to annexed content include the drive letter and full path, rather than
+    being relative. (`git annex fix` will fix up after this problem).
 
  -- Joey Hess <id@joeyh.name>  Tue, 24 Oct 2017 13:12:52 -0400
 

Utility/Path.hs

@@ -136,7 +136,9 @@ relPathDirToFile from to = relPathDirToFileAbs <$> absPath from <*> absPath to
  -}
 relPathDirToFileAbs :: FilePath -> FilePath -> FilePath
 relPathDirToFileAbs from to
-	| takeDrive from /= takeDrive to = to
+#ifdef mingw32_HOST_OS
+	| normdrive from /= normdrive to = to
+#endif
 	| otherwise = joinPath $ dotdots ++ uncommon
   where
 	pfrom = sp from
@@ -147,6 +149,9 @@ relPathDirToFileAbs from to
 	uncommon = drop numcommon pto
 	dotdots = replicate (length pfrom - numcommon) ".."
 	numcommon = length common
+#ifdef mingw32_HOST_OS
+	normdrive = map toLower . takeWhile (/= ':') . takeDrive
+#endif
 
 prop_relPathDirToFile_basics :: FilePath -> FilePath -> Bool
 prop_relPathDirToFile_basics from to

doc/bugs/windows_annex_link_wrong.mdwn

@@ -2,3 +2,5 @@ On windows, the links to .git/annex/objects are no longer relative; include
 drive letter and full path.
 
 This used to not be the case; it must have gotten broken. --[[Joey]]
+
+> [[fixed|done]]