fix test suite failure on windows

This was maybe a real bug too, although I don't know what circumstances
it would be a problem. See comment for analysis of this windows drive
letter wackyness issue.

Sponsored-by: Brock Spratlen on Patreon

Utility/Path.hs

@@ -187,7 +187,13 @@ relPathDirToFileAbs from to
 	dotdots = replicate (length pfrom - numcommon) ".."
 	numcommon = length common
 #ifdef mingw32_HOST_OS
-	normdrive = map toLower . takeWhile (/= ':') . fromRawFilePath . takeDrive
+	normdrive = map toLower
+		-- Get just the drive letter, removing any leading
+		-- path separator, which takeDrive leaves on the drive
+		-- letter.
+		. dropWhileEnd (isPathSeparator . fromIntegral . ord)
+		. fromRawFilePath 
+		. takeDrive
 #endif
 
 {- Checks if a command is available in PATH.

doc/bugs/test_prop__95__relPathDirToFileAbs__95__basics_fails_now__38__thn.mdwn

@@ -49,3 +49,5 @@ Windows version 21H1 (build 19043.1165), 64 bit.
 Git Annex is great. It works with multi-gigabyte backup files (largest around 180GB) via the BLAKE2B160E backend just dandy :)
 
 [[!meta author=jkniiv]]
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/test_prop__95__relPathDirToFileAbs__95__basics_fails_now__38__thn/comment_1_7becd972ce41d14ace89a9bc1302abba._comment

@@ -0,0 +1,28 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2021-09-01T14:54:56Z"
+ content="""
+Reproduced on Linux using System.FilePath.Windows.
+
+Minimal case is:
+
+	ghci> let p = "\\\\\DLEJ\STXm{u5;4*\EOTKo1"
+	ghci> relPathDirToFileAbs (p </> "bar") p
+	"\\\\\DLEJ\STXm{u5;4*\EOTKo1"
+
+Which should be "bar", but the "normdrive" case in
+relPathDirToFileAbs causes it to not return that.
+
+Ah, that whole value is treated as a "drive letter" due to starting
+with "\\\\" and not containing any path separator.
+
+	ghci> takeDrive p
+	"\\\\\DLEJ\STXm{u5;4*\EOTKo1"
+
+And takeDrive includes the first path separator, which is present in one
+string and not in another. So, it thinks these paths are on
+two different drives, when they are not. And that's the root
+of the problem. normdrive was working around that by taking up until 
+the ':', but there *is* no ':' in this drive letter!
+"""]]