upload-pack: refactor reading of pack-objects out

upload-pack: refactor reading of pack-objects out

Subsequent patches will change how the output of pack-objects is
processed, so extract that processing into its own function.

Currently, at most 1 character can be buffered (in the "buffered" local
variable). One of those patches will require a larger buffer, so replace
that "buffered" local variable with a buffer array.

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