Update to git version v2.20.0. Required changes follow upstream commits:

* 00436bf1b1c2a8fe6cf5d2c2457d419d683042f4
  (archive: initialize archivers earlier)

* 611e42a5980a3a9f8bb3b1b49c1abde63c7a191e
  (xdiff: provide a separate emit callback for hunks)

Signed-off-by: Christian Hesse <mail@eworm.de>