Make relative include entries relative to their own rc-file.

As opposed to making them relative to script location, original rc-file,
initial cwd, or similar. This allows for whole directory trees to be
moved around if need be as well as long as the internal links remain
consistent.

I've tested this on a sample here and also on the provided test case
with the bug and it seems to work well although I have not had time to
run through a whole build, merely the --pretend sequence.

BUG:329444
FIXED-IN:1.16