The old algorithm for determining the common prefix and trimming the folder name didn't work correctly when the glob result contained folder names that had the same prefix but then diverged. It then outputted just an empty folder name.

E.g.:
a/b-c/coverage.xml and a/b-d/coverage.xml would yield empty folder names for the separate paths.

I've instead opted to operate on path fragment level and comparing the folder names. This might subtly change the folder names the action outputs, e.g. no leading slash.

This also fixes #68.