git: update to v2.21.0

Update to git version v2.21.0. Required changes follow upstream commits:

* 6a7895fd8a3bd409f2b71ffc355d5142172cc2a0
  (commit: prepare free_commit_buffer and release_commit_memory for
  any repo)

* e092073d643b17c82d72cf692fbfaea9c9796f11
  (tree.c: make read_tree*() take 'struct repository *')

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 = $(prefix)/share/man
 SHA1_HEADER = <openssl/sha.h>
-GIT_VER = 2.20.0
+GIT_VER = 2.21.0
 GIT_URL = https://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.xz
 INSTALL = install
 COPYTREE = cp -r