spec: fix Requires/BuildRequires on git
For a long time we use 'dirxz' as ARCHIVE_FORMAT (we distribute
archive.dir.tar.xz) in fedora package. That's been this way
because 'xz' is historically installed by default in minimal
buildroot and because that's the preferred distribution method
(smallest size of the archive file).
Because 'dirxz' is preferred anyway, remove the redundant
--disable-git %configure option. But also to avoid surprises in
future, let's make the 'dirxz' explicit by having 'BuildRequires:
xz' and using '--enable-xz' configure option.
As it was just a redundant dependency, from now on we don't depend
Related: rhbz#1161284, rhbz#1416691