Smart-http: check if repository is OK to export before serving it

Smart-http: check if repository is OK to export before serving it

Similar to how git-daemon checks whether a repository is OK to be
exported, smart-http should also check.  This check can be satisfied
in two different ways: the environmental variable GIT_HTTP_EXPORT_ALL
may be set to export all repositories, or the individual repository
may have the file git-daemon-export-ok.

Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>