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>