switch from System.Cmd.Utils to System.Process

Test suite now passes with -threaded!

I traced back all the hangs with -threaded to System.Cmd.Utils. It seems
it's just crappy/unsafe/outdated, and should not be used. System.Process
seems to be the cool new thing, so converted all the code to use it
instead.

In the process, --debug stopped printing commands it runs. I may try to
bring that back later.

Note that even SafeSystem was switched to use System.Process. Since that
was a modified version of code from System.Cmd.Utils, it needed to be
converted too. I also got rid of nearly all calls to forkProcess,
and all calls to executeFile, which I'm also doubtful about working
well with -threaded.

Git/Branch.hs

@@ -76,9 +76,7 @@ commit message branch parentrefs repo = do
 	sha <- getSha "commit-tree" $ pipeWriteRead
 		(map Param $ ["commit-tree", show tree] ++ ps)
 		message repo
-	print ("got", sha)
 	run "update-ref" [Param $ show branch, Param $ show sha] repo
-	print ("update-ref done", sha)
 	return sha
 	where
 		ps = concatMap (\r -> ["-p", show r]) parentrefs