Merge branch 'bc/asciidoctor'

Add machinery to alternatively use AsciiDoctor to format our
documentation.

* bc/asciidoctor:
  Documentation: remove Asciidoctor linkgit macro
  Documentation: refactor common operations into variables
  Documentation: implement linkgit macro for Asciidoctor
  Documentation: move some AsciiDoc parameters into variables

Author: gitster

Documentation/Makefile

@@ -99,6 +99,13 @@ man7dir = $(mandir)/man7
 
 ASCIIDOC = asciidoc
 ASCIIDOC_EXTRA =
+ASCIIDOC_HTML = xhtml11
+ASCIIDOC_DOCBOOK = docbook
+ASCIIDOC_CONF = -f asciidoc.conf
+ASCIIDOC_COMMON = $(ASCIIDOC) $(ASCIIDOC_EXTRA) $(ASCIIDOC_CONF) \
+		-agit-version=$(GIT_VERSION)
+TXT_TO_HTML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_HTML)
+TXT_TO_XML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_DOCBOOK)
 MANPAGE_XSL = manpage-normal.xsl
 XMLTO = xmlto
 XMLTO_EXTRA =
@@ -306,14 +313,12 @@ clean:
 
 $(MAN_HTML): %.html : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b xhtml11 -d manpage -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
+	$(TXT_TO_HTML) -d manpage -o $@+ $< && \
 	mv $@+ $@
 
 $(OBSOLETE_HTML): %.html : %.txto asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b xhtml11 -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
+	$(TXT_TO_HTML) -o $@+ $< && \
 	mv $@+ $@
 
 manpage-base-url.xsl: manpage-base-url.xsl.in
@@ -325,13 +330,12 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 
 %.xml : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b docbook -d manpage -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
+	$(TXT_TO_XML) -d manpage -o $@+ $< && \
 	mv $@+ $@
 
 user-manual.xml: user-manual.txt user-manual.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b docbook -d article -o $@+ $< && \
+	$(TXT_TO_XML) -d article -o $@+ $< && \
 	mv $@+ $@
 
 technical/api-index.txt: technical/api-index-skel.txt \
@@ -340,8 +344,7 @@ technical/api-index.txt: technical/api-index-skel.txt \
 
 technical/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
 $(patsubst %,%.html,$(API_DOCS) technical/api-index $(TECH_DOCS)): %.html : %.txt asciidoc.conf
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) -b xhtml11 -f asciidoc.conf \
-		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) $*.txt
+	$(QUIET_ASCIIDOC)$(TXT_TO_HTML) $*.txt
 
 XSLT = docbook.xsl
 XSLTOPTS = --xinclude --stringparam html.stylesheet docbook-xsl.css
@@ -388,14 +391,15 @@ howto-index.txt: howto-index.sh $(wildcard howto/*.txt)
 	mv $@+ $@
 
 $(patsubst %,%.html,$(ARTICLES)) : %.html : %.txt
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 $*.txt
+	$(QUIET_ASCIIDOC)$(TXT_TO_HTML) $*.txt
 
 WEBDOC_DEST = /pub/software/scm/git/docs
 
 howto/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
 $(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	sed -e '1,/^$$/d' $< | $(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 - >$@+ && \
+	sed -e '1,/^$$/d' $< | \
+	$(TXT_TO_HTML) - >$@+ && \
 	mv $@+ $@
 
 install-webdoc : html