serve: provide "receive" function for object-format capability

serve: provide "receive" function for object-format capability

We get any "object-format" specified by the client by searching for it
in the collected list of capabilities the client sent. We can instead
just handle it as soon as they send it. This is slightly more efficient,
and gets us one step closer to dropping that collected list.

Note that we do still have to do our final hash check after receiving
all capabilities (because they might not have sent an object-format line
at all, and we still have to check that the default matches our
repository algorithm). Since the check_algorithm() function would now be
down to a single if() statement, I've just inlined it in its only
caller.

There should be no change of behavior here, except for two
broken-protocol cases:

  - if the client sends multiple conflicting object-format capabilities
    (which they should not), we'll now choose the last one rather than
    the first. We could also detect and complain about the duplicates
    quite easily now, which we could not before, but I didn't do so
    here.

  - if the client sends a bogus "object-format" with no equals sign,
    we'll now say so, rather than "unknown object format: ''"

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