Some firewalls restrict HTTP connections based on the clients user agent.  This
commit provides the user the ability to modify the user agent string via either
a new config option (http.useragent) or by an environment variable
(GIT_HTTP_USER_AGENT).

Relevant documentation is added to Documentation/config.txt.
Signed-off-by: NSpencer E. Olson <olsonse@umich.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>