Fix the installation path for html documentation

026fa0d (Move computation of absolute paths from Makefile to runtime in preparation for RUNTIME_PREFIX, 2009-01-18) broke the installation of html documentation. A relative htmldir is given to Documentation/Makefile and html documentations are installed in a subdirectory of "Documentation" in the source tree. Fix this by not exporting htmldir from Makefile; this allows Documentation/Makefile to compute the htmldir from the prefix. Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>