Revise rules for clobbering directories

In general, make directories behave like files:

1. Don't clobber a directory we don't know about.

2. Do clobber a directory if we do know about it.

This revises the behavior described in the following commits:

(1) commit f657dea
(2) commit 29925fe

I think it is a better solution to the problem mentioned in commit (1).
In particular, if we manually create a directory foo and then later try
to run foo.do, redo will warn that foo exists and is not generated, and
will not clobber the directory -- just as for a file.

Note that the distinction between files and directories mentioned in the
log for commit (1) (i.e., redo doesn't know about directories in the
same way as it knows about files) seems to be true of the original redo
implementation, but not intrinsic to the problem. I erased this
distinction.

lib/builder.py

@@ -3,7 +3,7 @@
 # Build redo targets
 # ======================================================================
 
-import sys, os, errno, stat, re
+import sys, os, errno, stat
 import vars, jobs, state, targets_seen, deps
 from helpers import remove, rename, close_on_exec, join
 from log import log, log_, debug, debug2, err, warn
@@ -169,8 +169,7 @@ def _start_do(self):
                 sf.set_checked()
                 sf.save()
                 return self._after2(0)
-        if (os.path.exists(t) and not os.path.isdir(t + '/.')
-             and not sf.is_generated):
+        if (os.path.exists(t) and not sf.is_generated):
             # an existing source file that was not generated by us.
             # This step is mentioned by djb in his notes.
             # For example, a rule called default.c.do could be used to try
@@ -352,17 +351,12 @@ def _after2(self, rv):
 # ----------------------------------------------------------------------
 
 def _remove(path):
-    redo_tmp = re.search('\.redo.\.tmp', path)
-    if not redo_tmp and os.path.isdir(path) and len(os.listdir(path)) > 0:
-        warn('directory %s is nonempty; not redoing\n' % _nice(path))
-        return False
-    else:
-        try:
-            remove(path)
-        except:
-            err('failed attempting to remove %s\n' % _nice(path))
-            raise
-        return True
+    try:
+        remove(path)
+    except:
+        err('failed attempting to remove %s\n' % _nice(path))
+        raise
+    return True
 
 
 def _rename(src, dest):