improve createDirectoryUnder to allow alternate top directories

This should not change the behavior of it, unless there are multiple top
directories, and then it should behave the same as if there was a single
top directory that was actually above the directory to be created.

Sponsored-by: Dartmouth College's Datalad project

Git/Repair.hs

@@ -269,7 +269,7 @@ explodePackedRefsFile r = do
 		let gitd = localGitDir r
 		let dest = gitd P.</> fromRef' ref
 		let dest' = fromRawFilePath dest
-		createDirectoryUnder gitd (parentDir dest)
+		createDirectoryUnder [gitd] (parentDir dest)
 		unlessM (doesFileExist dest') $
 			writeFile dest' (fromRef sha)