Update to git version v2.19.1. Required changes follow upstream commits:

* commit: add repository argument to get_cached_commit_buffer
  (3ce85f7e5a41116145179f0fae2ce6d86558d099)

* commit: add repository argument to lookup_commit_reference
  (2122f6754c93be8f02bfb5704ed96c88fc9837a8)

* object: add repository argument to parse_object
  (109cd76dd3467bd05f8d2145b857006649741d5c)

* tag: add repository argument to deref_tag
  (a74093da5ed601a09fa158e5ba6f6f14c1142a3e)

* tag: add repository argument to lookup_tag
  (ce71efb713f97f476a2d2ab541a0c73f684a5db3)

* tree: add repository argument to lookup_tree
  (f86bcc7b2ce6cad68ba1a48a528e380c6126705e)

* archive.c: avoid access to the_index
  (b612ee202a48f129f81f8f6a5af6cf71d1a9caef)

* for_each_*_object: move declarations to object-store.h
  (0889aae1cd18c1804ba01c1a4229e516dfb9fe9b)

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