Post 2.45.2 updates

Post 2.45.2 updates

Merge down a handful of topics to adjust tests and CI to make them
work better, without changing Git itself, and a bit of developer
docs update:

 * Tests that try to corrupt in-repository files in chunked format did
   not work well on macOS due to its broken "mv", which has been
   worked around.

 * Unbreak CI jobs so that we do not attempt to use Python 2 that has
   been removed from the platform.

 * Git 2.43 started using the tree of HEAD as the source of attributes
   in a bare repository, which has severe performance implications.
   For now, revert the change, without ripping out a more explicit
   support for the attr.tree configuration variable.

 * Windows CI running in GitHub Actions started complaining about the
   order of arguments given to calloc(); the imported regex code uses
   the wrong order almost consistently, which has been corrected.

Signed-off-by: Junio C Hamano <gitster@pobox.com>