t/lib-httpd: handle running under --valgrind

t/lib-httpd: handle running under --valgrind

Running the http tests with valgrind does not work for two
reasons:

  1. Apache complains about following the symbolic link from
     git-http-backend to valgrind.sh.

  2. Apache does not pass through the GIT_VALGRIND variable
     to the backend CGI.

This patch fixes both problems. Unfortunately, there is a
slight hack we need to handle passing environment variables
through Apache. If we just tell it:

  PassEnv GIT_VALGRIND

then Apache will complain when GIT_VALGRIND is not set. If
we try:

  SetEnv GIT_VALGRIND ${GIT_VALGRIND}

then when GIT_VALGRIND is not set, it will pass through the
literal "${GIT_VALGRIND}". Instead, we now unconditionally
pass through GIT_VALGRIND from lib-httpd.sh into apache,
even if it is empty.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>