git: update to v2.20.0

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>

Makefile

@@ -14,7 +14,7 @@ htmldir = $(docdir)
 pdfdir = $(docdir)
 mandir = $(prefix)/share/man
 SHA1_HEADER = <openssl/sha.h>
-GIT_VER = 2.19.1
+GIT_VER = 2.20.0
 GIT_URL = https://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.xz
 INSTALL = install
 COPYTREE = cp -r