Merge branch 'ls/travis-doc-asciidoctor' into maint

Travis CI gained a task to format the documentation with both
AsciiDoc and AsciiDoctor.

* ls/travis-doc-asciidoctor:
  travis-ci: check AsciiDoc/AsciiDoctor stderr output
  travis-ci: unset compiler for jobs that do not need one
  travis-ci: parallelize documentation build
  travis-ci: build documentation with AsciiDoc and Asciidoctor

Author: gitster

.travis.yml

@@ -57,6 +57,7 @@ matrix:
       after_failure:
     - env: Linux32
       os: linux
+      compiler:
       services:
         - docker
       before_install:
@@ -91,14 +92,14 @@ matrix:
       after_failure:
     - env: Documentation
       os: linux
-      compiler: clang
+      compiler:
       addons:
         apt:
           packages:
           - asciidoc
           - xmlto
       before_install:
-      before_script:
+      before_script: gem install asciidoctor
       script: ci/test-documentation.sh
       after_failure:
 

ci/test-documentation.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 #
 # Perform sanity checks on documentation and build it.
 #
@@ -7,8 +7,19 @@ set -e
 
 make check-builtins
 make check-docs
-make doc
 
+# Build docs with AsciiDoc
+make --jobs=2 doc > >(tee stdout.log) 2> >(tee stderr.log >&2)
+! test -s stderr.log
 test -s Documentation/git.html
 test -s Documentation/git.xml
 test -s Documentation/git.1
+grep '<meta name="generator" content="AsciiDoc ' Documentation/git.html
+
+# Build docs with AsciiDoctor
+make clean
+make --jobs=2 USE_ASCIIDOCTOR=1 doc > >(tee stdout.log) 2> >(tee stderr.log >&2)
+sed '/^GIT_VERSION = / d' stderr.log
+! test -s stderr.log
+test -s Documentation/git.html
+grep '<meta name="generator" content="Asciidoctor ' Documentation/git.html