doc: make HTML manual reproducible

Versions of docbook-xsl newer than 1.79.1 allows xsltproc to assign
IDs to nodes in the generated HTML consistently, to make the output
resulting from the same source stable and reproducible.

Pass the generate.consistent.ids parameter from the command line to
ask for this feature.  Older versions of the tool simply ignores the
parameter and produces their output the same way as before this
change, so there is no need to check for toolchain version.

Signed-off-by: Arnout Engelen <arnout@bzzt.net>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Helped-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -380,7 +380,10 @@ SubmittingPatches.txt: SubmittingPatches
 	$(QUIET_GEN) cp $< $@
 
 XSLT = docbook.xsl
-XSLTOPTS = --xinclude --stringparam html.stylesheet docbook-xsl.css
+XSLTOPTS =
+XSLTOPTS += --xinclude
+XSLTOPTS += --stringparam html.stylesheet docbook-xsl.css
+XSLTOPTS += --param generate.consistent.ids 1
 
 user-manual.html: user-manual.xml $(XSLT)
 	$(QUIET_XSLTPROC)$(RM) $@+ $@ && \