upload-pack: move 'struct upload_pack_data' around

upload-pack: move 'struct upload_pack_data' around

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, let's move 'struct upload_pack_data' and the
related upload_pack_data_init() and upload_pack_data_clear()
functions towards the beginning of the file, so that this struct
and its related functions can then be used by upload_pack() in a
followup commit.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>