use Asciidoctor to build documentation when possible

The Asciidoctor project is more actively maintained than asciidoc.  Use
it for building the documentation on Fedora.  Asciidoctor is not
currently available for EL-6 or EL-8, though it is in EPEL for EL-7.
Exclude all EL builds for now, until we can reliably use it on EL-7 and
EL-8 (including CentOS-Stream, ideally).

This is made possible by the excellent work of both the Git and
Asciidoctor communities.  Thanks in particular to brian m. carlson,
Martin Ågren, Jeff King, and Dan Allen.

git.spec

@@ -8,9 +8,11 @@
 
 # Settings for Fedora
 %if 0%{?fedora}
+%bcond_without              asciidoctor
 # linkchecker is not available on EL
 %bcond_without              linkcheck
 %else
+%bcond_with                 asciidoctor
 %bcond_with                 linkcheck
 %endif
 
@@ -83,7 +85,7 @@
 
 Name:           git
 Version:        2.25.1
-Release:        3%{?rcrev}%{?dist}
+Release:        4%{?rcrev}%{?dist}
 Summary:        Fast Version Control System
 License:        GPLv2
 URL:            https://git-scm.com/
@@ -118,7 +120,13 @@ Patch0:         git-cvsimport-Ignore-cvsps-2.2b1-Branches-output.patch
 %if %{with docs}
 # pod2man is needed to build Git.3pm
 BuildRequires:  %{_bindir}/pod2man
+%if %{with asciidoctor}
+BuildRequires:  docbook5-style-xsl
+BuildRequires:  rubygem-asciidoctor
+%else
 BuildRequires:  asciidoc >= 8.4.1
+%endif
+# endif with asciidoctor
 BuildRequires:  xmlto
 %if %{with linkcheck}
 BuildRequires:  linkchecker
@@ -526,6 +534,10 @@ PYTHON_PATH = %{__python2}
 NO_PYTHON = 1
 %endif
 # endif with python2
+%if %{with asciidoctor}
+USE_ASCIIDOCTOR = 1
+%endif
+# endif with asciidoctor
 htmldir = %{?_pkgdocdir}%{!?_pkgdocdir:%{_docdir}/%{name}-%{version}}
 prefix = %{_prefix}
 perllibdir = %{perl_vendorlib}
@@ -1033,6 +1045,9 @@ rmdir --ignore-fail-on-non-empty "$testdir"
 %{?with_docs:%{_pkgdocdir}/git-svn.html}
 
 %changelog
+* Wed Feb 26 2020 Todd Zullinger <tmz@pobox.com> - 2.25.1-4
+- use Asciidoctor to build documentation when possible
+
 * Sat Feb 22 2020 Todd Zullinger <tmz@pobox.com> - 2.25.1-3
 - work around issue on s390x with gcc10 (#1799408)