Allow HTTP user agent string to be modified.

Allow HTTP user agent string to be modified.

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: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>