git: update to v2.22.0

Update to git version v2.22.0.

Upstream commit bce9db6d ("trace2: use system/global config for default
trace2 settings") caused a regression. We have to unset HOME and
XDG_CONFIG_HOME before early loading of config from trace2 code kicks in.

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

Makefile

@@ -14,7 +14,7 @@ htmldir = $(docdir)
 pdfdir = $(docdir)
 mandir = $(prefix)/share/man
 SHA1_HEADER = <openssl/sha.h>
-GIT_VER = 2.21.0
+GIT_VER = 2.22.0
 GIT_URL = https://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.xz
 INSTALL = install
 COPYTREE = cp -r