Documentation: refactor common operations into variables

The Makefile performs several very similar tasks to convert AsciiDoc
files into either HTML or DocBook.  Move these items into variables to
reduce the duplication.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -100,6 +100,10 @@ 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 =
@@ -307,14 +311,12 @@ clean:
 
 $(MAN_HTML): %.html : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b $(ASCIIDOC_HTML) -d manpage $(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 $(ASCIIDOC_HTML) $(ASCIIDOC_CONF) \
-		$(ASCIIDOC_EXTRA) -agit_version=$(GIT_VERSION) -o $@+ $< && \
+	$(TXT_TO_HTML) -o $@+ $< && \
 	mv $@+ $@
 
 manpage-base-url.xsl: manpage-base-url.xsl.in
@@ -326,13 +328,12 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 
 %.xml : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(ASCIIDOC) -b $(ASCIIDOC_DOCBOOK) -d manpage $(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 $(ASCIIDOC_DOCBOOK) -d article -o $@+ $< && \
+	$(TXT_TO_XML) -d article -o $@+ $< && \
 	mv $@+ $@
 
 technical/api-index.txt: technical/api-index-skel.txt \
@@ -341,8 +342,7 @@ 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 $(ASCIIDOC_HTML) $(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
@@ -389,15 +389,15 @@ howto-index.txt: howto-index.sh $(wildcard howto/*.txt)
 	mv $@+ $@
 
 $(patsubst %,%.html,$(ARTICLES)) : %.html : %.txt
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b $(ASCIIDOC_HTML) $*.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 $(ASCIIDOC_HTML) - >$@+ && \
+	$(TXT_TO_HTML) - >$@+ && \
 	mv $@+ $@
 
 install-webdoc : html