`base` metadata value is ignored

[dkaumanns]

With Pandoc 2.19.2, the base metadata values seems to have no effect:

$ pandoc [...] --metadata=base:myarticledirectory

On:

~~~{include=src/algorithms.cpp}
~~~

Results in:

pandoc-include-code: src/algorithms.cpp: openFile: does not exist (No such file or directory)

Explizit code block include works:

~~~{include=myarticledirectory/src/algorithms.cpp}
~~~

Thank you for this tool.