debian/rules: skip html generation when DEB_BUILD_OPTIONS=nodoc

debian/rules: skip html generation when DEB_BUILD_OPTIONS=nodoc

Debian policy 4.9.1 explains:

	nodoc
		This tag says to skip any build steps that only
		generate package documentation. Files required by
		other sections of Debian Policy, such as copyright and
		changelog files, must still be generated and put in
		the package, but other generated documentation such as
		help2man-generated pages, Doxygen-generated API
		documentation, or info pages generated from Texinfo
		sources should be skipped if possible. This option
		does not change the set of binary packages generated
		by the source package, but documentation-only binary
		packages may be nearly empty when built with this
		option.

For now this will only affect HTML documentation, not the man pages in
git-man, since those are needed for 'git <command> --help' to work.

Tested by running "DEB_BUILD_OPTIONS=nodoc fakeroot debian/rules binary"
and examining the result.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>