connect.c: remove a few globals by using git_config callback data

connect.c: remove a few globals by using git_config callback data

Since ef90d6d (Provide git_config with a callback-data parameter,
2008-05-14), git_config() takes a callback data pointer that can be
used to pass extra parameters to the parsing function.  The codepath
to parse configuration variables related to git proxy predates this
facility and used a pair of file scope static variables instead.

This patch removes the need for these global variables by passing the
name of the host we are trying to access as the callback data.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>