git: update to v2.18.0

Update to git version v2.18.0. Required changes follow upstream commits:

* Convert find_unique_abbrev* to struct object_id
  (aab9583f7b5ea5463eb3f653a0b4ecac7539dc94)
* sha1_file: convert read_sha1_file to struct object_id
  (b4f5aca40e6f77cbabcbf4ff003c3cf30a1830c8)
* sha1_file: convert sha1_object_info* to object_id
  (abef9020e3df87c441c9a3a95f592fce5fa49bb9)
* object-store: move packed_git and packed_git_mru to object store
  (a80d72db2a73174b3f22142eb2014b33696fd795)
* treewide: rename tree to maybe_tree
  (891435d55da80ca3654b19834481205be6bdfe33)

The changed data types required some of our own functions to be converted
to struct object_id:

  ls_item
  print_dir
  print_dir_entry
  print_object
  single_tree_cb
  walk_tree
  write_tree_link

And finally we use new upstream functions that were added for
struct object_id:

  hashcpy     -> oidcpy
  sha1_to_hex -> oid_to_hex

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>

Makefile

@@ -14,7 +14,7 @@ htmldir = $(docdir)
 pdfdir = $(docdir)
 mandir = $(prefix)/share/man
 SHA1_HEADER = <openssl/sha.h>
-GIT_VER = 2.17.1
+GIT_VER = 2.18.0
 GIT_URL = https://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.gz
 INSTALL = install
 COPYTREE = cp -r