fix: don't capture STDIN without interactive or use_stdin options (#638)

If I'm reading the code, this `args.in` should be a nullReader when
non-interactive and not using stdin. For some reason though, it ends up
capturing stdin. That is, any typing you do while lefthook is running
goes to lefthook, instead of the shell invoking it.

Fixes #588

internal/lefthook/run/exec/execute_unix.go

@@ -102,8 +102,6 @@ func (e CommandExecutor) execute(ctx context.Context, cmdstr string, args *execu
 
 		defer func() { _ = p.Close() }()
 
-		go func() { _, _ = io.Copy(p, args.in) }()
-
 		_, _ = io.Copy(args.out, p)
 	}