improve gpg verification of upstream source

Use %{gpgverify} macro to verify tarball signature.  The macro is now
available for all supported Fedora and EPEL releases.  (It is presumed
that EL-9 will include %{gpgverify} as it will be branched from F-34.
If that turns out to be false, we will adjust later.)

The Packaging Guidelines require the use of the %{gpgverify} macro:

    https://docs.fedoraproject.org/en-US/packaging-guidelines/#_verifying_signatures

Add a BuildRequires for xz as well, since we use it explicitly in %prep.

Renumber Junio's GPG key from Source9 to Source2 so the %{gpgverify}
calls follow the typical pattern.  It (mildly) lessens cognitive load
for anyone reviewing the spec file.

While here, remove a stale comment about leaving a blank line after
%autosetup to work around a bug on EL6.

git.spec

@@ -97,7 +97,7 @@
 
 Name:           git
 Version:        2.30.1
-Release:        2%{?rcrev}%{?dist}.1
+Release:        3%{?rcrev}%{?dist}
 Summary:        Fast Version Control System
 License:        GPLv2
 URL:            https://git-scm.com/
@@ -113,7 +113,7 @@ Source1:        https://www.kernel.org/pub/software/scm/git/%{?rcrev:testing/}%{
 #
 # https://git.kernel.org/cgit/git/git.git/tag/?h=junio-gpg-pub
 # https://git.kernel.org/cgit/git/git.git/blob/?h=junio-gpg-pub&id=7214aea37915ee2c4f6369eb9dea520aec7d855b
-Source9:        gpgkey-junio.asc
+Source2:        gpgkey-junio.asc
 
 # Local sources begin at 10 to allow for additional future upstream sources
 Source11:       git.xinetd.in
@@ -152,11 +152,12 @@ BuildRequires:  diffutils
 BuildRequires:  emacs-common
 %endif
 # endif emacs-common
-%if 0%{?rhel} == 7
-# Require epel-rpm-macros for the %%build_cflags and %%build_ldflags macros
+%if 0%{?rhel} && 0%{?rhel} < 9
+# Require epel-rpm-macros for the %%gpgverify macro on EL-7/EL-8, and
+# %%build_cflags & %%build_ldflags on EL-7.
 BuildRequires:  epel-rpm-macros
 %endif
-# endif rhel == 7
+# endif rhel < 9
 BuildRequires:  expat-devel
 BuildRequires:  findutils
 BuildRequires:  gawk
@@ -192,6 +193,7 @@ BuildRequires:  systemd
 # endif use_systemd
 BuildRequires:  tcl
 BuildRequires:  tk
+BuildRequires:  xz
 BuildRequires:  zlib-devel >= 1.2
 
 %if %{with tests}
@@ -521,16 +523,8 @@ Requires:       subversion
 
 %prep
 # Verify GPG signatures
-gpghome="$(mktemp -qd)" # Ensure we don't use any existing gpg keyrings
-# Convert the ascii-armored key to binary
-# (use --yes to ensure an existing dearmored key is overwritten)
-gpg2 --homedir "$gpghome" --dearmor --quiet --yes %{SOURCE9}
-xz -dc %{SOURCE0} | # Upstream signs the uncompressed tarballs
-    gpgv2 --homedir "$gpghome" --quiet --keyring %{SOURCE9}.gpg %{SOURCE1} -
-rm -rf "$gpghome" # Cleanup tmp gpg home dir
+xz -dc '%{SOURCE0}' | %{gpgverify} --keyring='%{SOURCE2}' --signature='%{SOURCE1}' --data=-
 
-# Ensure a blank line follows autosetup, el6 chokes otherwise
-# https://bugzilla.redhat.com/1310704
 %autosetup -p1 -n %{name}-%{version}%{?rcrev}
 
 # Install print-failed-test-output script
@@ -1076,6 +1070,9 @@ rmdir --ignore-fail-on-non-empty "$testdir"
 %{?with_docs:%{_pkgdocdir}/git-svn.html}
 
 %changelog
+* Tue Mar 02 2021 Todd Zullinger <tmz@pobox.com> - 2.30.1-3
+- use %%{gpgverify} macro to verify tarball signature
+
 * Tue Mar 02 2021 Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> - 2.30.1-2.1
 - Rebuilt for updated systemd-rpm-macros
   See https://pagure.io/fesco/issue/2583.