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.

t/370-logs/x.do

@@ -0,0 +1,5 @@
+read pid <pid
+echo "$pid x stdout"
+echo "$pid x stderr" >&2
+rm -f y
+redo y