doc: consistently use ASCIIDOC_EXTRA

For all uses of $(ASCIIDOC) in Documentation/Makefile, supply the same
options via $(ASCIIDOC_EXTRA).

Signed-off-by: Eric Blake <ebb9@byu.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -240,7 +240,7 @@ $(MAN_HTML): %.html : %.txt
 	mv $@+ $@
 
 user-manual.xml: user-manual.txt user-manual.conf
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) -b docbook -d book $<
+	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b docbook -d book $<
 
 technical/api-index.txt: technical/api-index-skel.txt \
 	technical/api-index.sh $(patsubst %,%.txt,$(API_DOCS))
@@ -293,13 +293,13 @@ howto-index.txt: howto-index.sh $(wildcard howto/*.txt)
 	mv $@+ $@
 
 $(patsubst %,%.html,$(ARTICLES)) : %.html : %.txt
-	$(QUIET_ASCIIDOC)$(ASCIIDOC) -b xhtml11 $*.txt
+	$(QUIET_ASCIIDOC)$(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 $*.txt
 
 WEBDOC_DEST = /pub/software/scm/git/docs
 
 $(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	sed -e '1,/^$$/d' $< | $(ASCIIDOC) -b xhtml11 - >$@+ && \
+	sed -e '1,/^$$/d' $< | $(ASCIIDOC) $(ASCIIDOC_EXTRA) -b xhtml11 - >$@+ && \
 	mv $@+ $@
 
 install-webdoc : html