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.

redo-sources.py

@@ -0,0 +1,18 @@
+#!/usr/bin/python
+import sys, os
+
+import vars_init
+vars_init.init([])
+
+import state
+from log import err
+
+if len(sys.argv[1:]) != 0:
+    err('%s: no arguments expected.\n' % sys.argv[0])
+    sys.exit(1)
+
+for f in state.files():
+    if f.name.startswith('//'):
+        continue  # special name, ignore
+    if not f.is_generated and f.read_stamp() != state.STAMP_MISSING:
+        print f.nicename()