Allow help.htmlpath to be a URL prefix

Allow help.htmlpath to be a URL prefix

Setting this to a URL prefix instead of a path to a local directory allows
git-help --web to work even when HTML docs aren't locally installed, by
pointing the browser at a copy accessible on the web. For example,

    [help]
      format = html
      htmlpath = http://git-scm.com/docs

will use the publicly available documentation on the git homepage.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>