minimal/do: add redo-whichdo support and internal unit tests.

Adding redo-whichdo wasn't so hard, except that it exposed a bunch of
mistakes in the way minimal/do was choosing .do files.  Most of the
wrong names it tried didn't matter (since they're very unlikely to
exist) except that they produce the wrong redo-whichdo output.

Debugging that took so much effort that I added unit tests for some
functions to make it easier to find problems in the future.

Note: this change apparently makes minimal/do about 50% slower than
before, because the _find_dofiles algorithm got less optimized (but
more readable).  It's still not very slow, and anyway, since it's
minimal/do, I figured readable/correct code probably outweighed a bit
of speed.  (Although if anyone can come up with an algorithm that's
clear *and* works better, I won't turn it down :)  I feel like I must
be doing it the hard way...)

test.do

@@ -1,3 +1,3 @@
 redo-ifchange _all
-redo t/all
+redo minimal/test t/all
 [ -n "$DO_BUILT" ] || echo "Don't forget to test 'minimal/do -c test'" >&2