Documentation: move dblatex arguments into variable

Our dblatex invocation uses several style components from the AsciiDoc
distribution, but those components are not available when building with
Asciidoctor.  Move the command line arguments into a variable so it can
be overridden by the user or makefile configuration options.

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

Documentation/Makefile

@@ -120,6 +120,7 @@ INSTALL_INFO = install-info
 DOCBOOK2X_TEXI = docbook2x-texi
 DBLATEX = dblatex
 ASCIIDOC_DBLATEX_DIR = /etc/asciidoc/dblatex
+DBLATEX_COMMON = -p $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.xsl -s $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.sty
 ifndef PERL_PATH
 	PERL_PATH = /usr/bin/perl
 endif
@@ -368,7 +369,7 @@ user-manual.texi: user-manual.xml
 
 user-manual.pdf: user-manual.xml
 	$(QUIET_DBLATEX)$(RM) $@+ $@ && \
-	$(DBLATEX) -o $@+ -p $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.xsl -s $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.sty $< && \
+	$(DBLATEX) -o $@+ $(DBLATEX_COMMON) $< && \
 	mv $@+ $@
 
 gitman.texi: $(MAN_XML) cat-texi.perl texi.xsl