upload-pack: clear flags before each v2 request

upload-pack: clear flags before each v2 request

Suppose a server has the following commit graph:

 A   B
  \ /
   O

We create a client by cloning A from the server with depth 1, and add
many commits to it (so that future fetches span multiple requests due to
lengthy negotiation). If it then fetches B using protocol v2, the fetch
spanning multiple requests, the resulting packfile does not contain O
even though the client did report that A is shallow.

This is because upload_pack_v2() can be called multiple times while
processing the same session. During the 2nd and all subsequent
invocations, some object flags remain from the previous invocations. In
particular, CLIENT_SHALLOW remains, preventing process_shallow() from
adding client-reported shallows to the "shallows" array, and hence
pack-objects not knowing about these client-reported shallows.

Therefore, teach upload_pack_v2() to clear object flags at the start of
each invocation. This has some other results:

 - THEY_HAVE gates addition of objects to have_obj in process_haves().
   Previously in upload_pack_v2(), have_obj needed to be static because
   once an object is added to have_obj, it is never readded and thus we
   needed to retain the contents of have_obj between invocations. Now
   that flags are cleared, this is no longer necessary. This patch does
   not change the behavior of ok_to_give_up() (THEY_HAVE is still set on
   each "have") and got_oid() (used only in non-v2)); THEY_HAVE is not
   used in any other function.

 - WANTED gates addition of objects to want_obj in parse_want() and
   parse_want_ref(). It is also used in receive_needs(), but that is
   only used in non-v2. For the same reasons as THEY_HAVE, want_obj no
   longer needs to be static in upload_pack_v2().

 - CLIENT_SHALLOW is changed as discussed above.

Clearing of the other 5 flags does not affect functionality in v2. (Note
that in non-v2, upload_pack() is only called once per process, so each
invocation starts with blank flags anyway.)

 - OUR_REF is only used in non-v2.

 - COMMON_KNOWN is only used as a scratch flag in ok_to_give_up().

 - SHALLOW is passed to invocations in deepen() and
   deepen_by_rev_list(), but upload-pack doesn't use it.

 - NOT_SHALLOW is used by send_shallow() and send_unshallow(), but
   invocations of those functions are always preceded by code that sets
   NOT_SHALLOW on the appropriate objects.

 - HIDDEN_REF is only used in non-v2.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>