Makefile: use more reliable git tarball mirror

Makefile

@@ -15,7 +15,7 @@ pdfdir = $(docdir)
 mandir = $(prefix)/share/man
 SHA1_HEADER = <openssl/sha.h>
 GIT_VER = 1.9.1
-GIT_URL = https://git-core.googlecode.com/files/git-$(GIT_VER).tar.gz
+GIT_URL = https://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.gz
 INSTALL = install
 COPYTREE = cp -r
 MAN5_TXT = $(wildcard *.5.txt)