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.

libdo.py

@@ -0,0 +1,27 @@
+import sys, os
+import vars
+
+def relpath(t, base):
+    t = os.path.abspath(t)
+    tparts = t.split('/')
+    bparts = base.split('/')
+    for tp,bp in zip(tparts,bparts):
+        if tp != bp:
+            break
+        tparts.pop(0)
+        bparts.pop(0)
+    while bparts:
+        tparts.insert(0, '..')
+        bparts.pop(0)
+    return '/'.join(tparts)
+
+
+def sname(typ, t):
+    # FIXME: t.replace(...) is non-reversible and non-unique here!
+    tnew = relpath(t, vars.BASE)
+    #log('sname: (%r) %r -> %r\n' % (vars.BASE, t, tnew))
+    return vars.BASE + ('/.redo/%s^%s' % (typ, tnew.replace('/', '^')))
+
+
+def add_dep(t, mode, dep):
+    open(sname('dep', t), 'a').write('%s %s\n' % (mode, dep))