New redo-sources and redo-targets commands.

Suggested by djb in personal email, and on the mailing list.  redo-targets
lists all the targets in the database; redo-sources lists all the existing
sources (ie. files that are referred to but which aren't targets).

redo-ifcreate filenames aren't included in the redo-sources list.

helpers.py

@@ -2,6 +2,10 @@ import os, errno, fcntl
 from atoi import atoi
 
 
+def join(between, l):
+    return between.join(l)
+
+
 def unlink(f):
     """Delete a file at path 'f' if it currently exists.