redo-log: add automated tests, and fix some path bugs revealed by them.

When a log for X was saying it wanted to refer to Y, we used a relative
path, but it was sometimes relative to the wrong starting location, so
redo-log couldn't find it later.

Two examples:

 - if default.o.do is handling builds for a/b/x.o, and default.o.do
   does 'redo a/b/x.h', the log for x.o should refer to ./x.h, not
   a/b/x.h.

 - if foo.do is handling builds for foo, and it does
   "cd a/b && redo x", the log for foo should refer to a/b/x, not just
   x.

state.py

@@ -165,6 +165,21 @@ def relpath(t, base):
     return join('/', tparts)
 
 
+# Return a path for t, if cwd were the dirname of vars.TARGET.
+# This is tricky!  STARTDIR+PWD is the directory for the *dofile*, when
+# the dofile was started.  However, inside the dofile, someone may have done
+# a chdir to anywhere else.  vars.TARGET is relative to the dofile path, so
+# we have to first figure out where the dofile was, then find TARGET relative
+# to that, then find t relative to that.
+#
+# FIXME: find some cleaner terminology for all these different paths.
+def target_relpath(t):
+    dofile_dir = os.path.abspath(os.path.join(vars.STARTDIR, vars.PWD))
+    target_dir = os.path.abspath(
+        os.path.dirname(os.path.join(dofile_dir, vars.TARGET)))
+    return relpath(t, target_dir)
+
+
 def warn_override(name):
     warn('%s - you modified it; skipping\n' % name)