Using this older version is not in general secure, since the new
version is a security update. It is sometimes acceptable to run
software with security bugs in CI workflows, but the intent of this
change is just to check if the version of the Cygwin `git` package
is the cause of the failures. If so, they can probably be fixed or
worked around in a better way than downgrading. (Furthermore, the
lower version of the `git` package will not always be avaialable
from Cygwin's repositories.)

This undoes the change of pinning Git to an earlier version (before
the recent security update) on Cygwin, and instead adds the `.git`
subdirectory of the `GitPython` directory as an additional value of
the multi-valued `safe.directory` Git configuration variable.