Set the parallel HTTP request limit via an environment variable

Use an environment variable rather than a command-line argument to set the
parallel HTTP request limit.  This allows the setting to work whether
git-http-fetch is run directly or via git-fetch.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>