Fix the installation path for html documentation

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>