Windows: Fix handling of absolute unix-style git repository paths.

Note that on Windows a remote with a path like /home/foo/bar
is interpreted by git as being some screwy relative path (relative to what
exactly seems ill-defined -- it seemed relative to C:\Program Files\Git\ in
my tests!) So no attempt has been made to handle such a path sanely, just not
to crash when encountering it.

Note that "C:\\foo" </> "/home/foo/bar" yields /home/foo/bar even though
that is not absolute! I don't know what to make of all this,
except that I will be very happy when this crock of **** vanishes from
the face of the earth.

Git/FilePath.hs

@@ -20,12 +20,15 @@ module Git.FilePath (
 	asTopFilePath,
 	InternalGitPath,
 	toInternalGitPath,
-	fromInternalGitPath
+	fromInternalGitPath,
+	absoluteGitPath
 ) where
 
 import Common
 import Git
 
+import qualified System.FilePath.Posix
+
 {- A FilePath, relative to the top of the git repository. -}
 newtype TopFilePath = TopFilePath { getTopFilePath :: FilePath }
 	deriving (Show)
@@ -66,3 +69,10 @@ fromInternalGitPath = id
 #else
 fromInternalGitPath = replace "/" "\\"
 #endif
+
+{- isAbsolute on Windows does not think "/foo" or "\foo" is absolute,
+ - so try posix paths.
+ -}
+absoluteGitPath :: FilePath -> Bool
+absoluteGitPath p = isAbsolute p ||
+	System.FilePath.Posix.isAbsolute (toInternalGitPath p)