add back debug logging

Make Utility.Process wrap the parts of System.Process that I use,
and add debug logging to them.

Also wrote some higher-level code that allows running an action
with handles to a processes stdin or stdout (or both), and checking
its exit status, all in a single function call.

As a bonus, the debug logging now indicates whether the process
is being run to read from it, feed it data, chat with it (writing and
reading), or just call it for its side effect.

Git/Config.hs

@@ -9,7 +9,7 @@ module Git.Config where
 
 import qualified Data.Map as M
 import Data.Char
-import System.Process
+import System.Process (cwd)
 
 import Common
 import Git
@@ -48,14 +48,11 @@ read' repo = go repo
 		go Repo { location = Local { gitdir = d } } = git_config d
 		go Repo { location = LocalUnknown d } = git_config d
 		go _ = assertLocal repo $ error "internal"
-		git_config d = do
-			(_, Just h, _, pid)
-				<- createProcess (proc "git" params)
-					{ std_out = CreatePipe, cwd = Just d }
-			repo' <- hRead repo h
-			forceSuccessProcess pid "git" params
-			return repo'
-		params = ["config", "--null", "--list"]
+		git_config d = withHandle StdoutHandle createProcessSuccess p $
+			hRead repo
+			where
+				params = ["config", "--null", "--list"]
+				p = (proc "git" params) { cwd = Just d }
 
 {- Reads git config from a handle and populates a repo with it. -}
 hRead :: Repo -> Handle -> IO Repo