the context directory is lost in nested requirements files

I have a directory with requirements files, where one file reference the others, for example:
- requirements
  - all.txt
  - foo.txt
  - bar.txt

where all.txt is:
-r foo.txt
-r bar.txt

When calling pbr.packaging.parse_requirements(['requirements/all.txt']), pbr loses the context directory and will try to look for foo.txt in the root directory instead of the requirements directory.