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.
