fedora: Define git_hash in spec file and reuse it

...as it's used twice. The short version, however, appears hardcoded
only once in the output, and it comes straight from the rpkg macro
building the version string -- leave that macro as it is.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

contrib/fedora/passt.spec

@@ -7,6 +7,8 @@
 # Copyright (c) 2022 Red Hat GmbH
 # Author: Stefano Brivio <sbrivio@redhat.com>
 
+%global git_hash {{{ git_head }}}
+
 Name:		passt
 Version:	{{{ git_version }}}
 Release:	1%{?dist}
@@ -14,7 +16,7 @@ Summary:	User-mode networking daemons for virtual machines and namespaces
 License:	AGPLv3+ and BSD
 Group:		System Environment/Daemons
 URL:		https://passt.top/
-Source:		https://passt.top/passt/snapshot/passt-{{{ git_head }}}.tar.xz
+Source:		https://passt.top/passt/snapshot/passt-%{git_hash}.tar.xz
 
 BuildRequires:	gcc, make, checkpolicy, selinux-policy-devel
 
@@ -40,7 +42,7 @@ Requires(preun): policycoreutils, %{name}
 This package adds SELinux enforcement to passt(1) and pasta(1).
 
 %prep
-%setup -q -n passt-{{{ git_head }}}
+%setup -q -n passt-%{git_hash}
 
 %build
 %set_build_flags