Move 'redo --ifchange' into 'redo-ifchange' to match djb's style.

It does simplify the logic of both redo.py and redo-ifchange.py, I suppose.

redo-ifchange.py

@@ -0,0 +1,61 @@
+#!/usr/bin/python
+import sys, os
+import vars
+from helpers import *
+from log import *
+from libdo import *
+
+
+def _dirty_deps(t, depth):
+    debug('%s?%s\n' % (depth, t))
+    if not os.path.exists(sname('stamp', t)):
+        debug('%s-- DIRTY (no stamp)\n' % depth)
+        return True
+
+    stamptime = os.stat(sname('stamp', t)).st_mtime
+    try:
+        realtime = os.stat(t).st_mtime
+    except OSError:
+        realtime = 0
+
+    if stamptime != realtime:
+        debug('%s-- DIRTY (mtime)\n' % depth)
+        return True
+    
+    for sub in open(sname('dep', t)).readlines():
+        assert(sub[0] in ('c','m'))
+        assert(sub[1] == ' ')
+        assert(sub[-1] == '\n')
+        mode = sub[0]
+        name = sub[2:-1]
+        if mode == 'c':
+            if os.path.exists(name):
+                debug('%s-- DIRTY (created)\n' % depth)
+                return True
+        elif mode == 'm':
+            if dirty_deps(name, depth + '  '):
+                #debug('%s-- DIRTY (sub)\n' % depth)
+                return True
+    return False
+
+
+def dirty_deps(t, depth):
+    if _dirty_deps(t, depth):
+        unlink(sname('stamp', t))  # short circuit future checks
+        return True
+    return False
+
+
+if not vars.TARGET:
+    sys.stderr.write('redo-ifchange: error: must be run from inside a .do\n')
+    sys.exit(1)
+
+want_build = []
+for t in sys.argv[1:]:
+    mkdirp('%s/.redo' % vars.BASE)
+    add_dep(vars.TARGET, 'm', t)
+    if dirty_deps(t, depth = ''):
+        want_build.append(t)
+
+if want_build:
+    os.execvp('redo', ['redo', '--'] + want_build)