Empty coverage report depending on mypy call location

hi,

to my understanding, there's a bug in mypy coverage report generation.
talking about the --html-report and --xml-report options (haven't tested the others).
if you call mypy from a folder parallel to the location of your sources folder, the files are analyzed properly but the report is empty.
no error is showing

steps to reproduce :

• create a dummy module for mypy to analyse
• create a folder next to your sources folder
• call mypy ../src/mod/myfile.py --xml-report empty-report
• look a the file index.xml and see that myfile.py is not listed

conversely if you call mypy from the source folder, you get a proper report

see attached files for an example.

mypy-report-bug.zip

Riproduci il problema di generazione dei report eseguendo mypy ../src/mod/myfile.py --xml-report empty-report da una cartella parallela alla cartella sorgente, quindi confrontalo con un'esecuzione dalla cartella sorgente. Inizia dagli entry point --xml-report e --html-report e ispeziona index.xml; il lavoro è completato quando il file analizzato appare nel report generato in entrambe le posizioni.