http-push: free curl header lists

http-push: free curl header lists

To pass headers to curl, we have to allocate a curl_slist linked list
and then feed it to curl_easy_setopt(). But the header list is not
copied by curl, and must remain valid until we are finished with the
request.

A few spots in http-push get this right, freeing the list after
finishing the request, but many do not. In most cases the fix is simple:
we set up the curl slot, start it, and then use run_active_slot() to
take it to completion. After that, we don't need the headers anymore and
can call curl_slist_free_all().

But one case is trickier: when we do a MOVE request, we start the
request but don't immediately finish it. It's possible we could change
this to be more like the other requests, but I didn't want to get into
risky refactoring of this code. So we need to stick the header list into
the request struct and remember to free it later.

Curiously, the struct already has a headers field for this purpose! It
goes all the way back to 58e60dd203 (Add support for pushing to a remote
repository using HTTP/DAV, 2005-11-02), but it doesn't look like it was
ever used. We can make use of it just by assigning our headers to it,
and there is already code in finish_request() to clean it up.

This fixes several leaks triggered by t5540.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>