git: update to v2.7.0

Update to git version v2.7.0.

* Upstream commit ed1c9977cb1b63e4270ad8bdf967a2d02580aa08 (Remove
  get_object_hash.) changed API:

  Convert all instances of get_object_hash to use an appropriate
  reference to the hash member of the oid member of struct object.
  This provides no functional change, as it is essentially a macro
  substitution.

Signed-off-by: Christian Hesse <mail@eworm.de>
diff --git a/ui-tree.c b/ui-tree.c
index d71503d..3ff2320 100644
--- a/ui-tree.c
+++ b/ui-tree.c
@@ -287,7 +287,7 @@
 	walk_tree_ctx.curr_rev = xstrdup(rev);
 
 	if (path == NULL) {
-		ls_tree(commit->tree->object.sha1, NULL, &walk_tree_ctx);
+		ls_tree(commit->tree->object.oid.hash, NULL, &walk_tree_ctx);
 		goto cleanup;
 	}