http-push: send out fetch requests on queue

http-push: send out fetch requests on queue

Previously, requests for remote files were simply added to the queue
(pointed to by request_queue_head) and no transfer actually takes
place (the fill function add_fill_function() is not added until line
2441), even though code that followed may rely on these remote files to
be present (eg. the setup_revisions invocation).

The code that sends out the requests on the request queue is refactored
into the method run_request_queue.

After the get_dav_remote_heads invocation (ie. after fetch requests are
added to the queue), the requests on the queue are sent out through an
invocation to run_request_queue.

This invocation to run_request_queue entails adding a fill function
before pushing checks take place, which may lead to accidental,
unwanted pushes previously.

The flag is_running_queue is introduced to prevent this from occurring.
fill_active_slot is made to check the flag is_running_queue before
the sending of the requests proceeds.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>