upload-pack: send ERR packet for non-tip objects

upload-pack: send ERR packet for non-tip objects

Commit bdb31eada7 (upload-pack: report "not our ref" to client,
2017-02-23) catches the case where a client asks for an object we don't
have, and issues a message that the client can show to the user (in
addition to dying and writing to stderr).

There's a similar case (with the same message) when the client asks for
an object which we _do_ have, but which isn't a ref tip (or isn't
reachable, when uploadpack.allowReachableSHA1InWant is true). Let's give
that one the same treatment, for the same reason (namely that it's more
informative to the client than just hanging up, since they won't see our
stderr over some protocols).

There are two tests here. We cover it most directly in t5530 by invoking
upload-pack, which matches the existing "not our ref" test.

But a more end-to-end check is that "git fetch" actually shows the
message to the client. We're already checking in t5516 that this case
fails, so we can just check stderr there, too. Note that even after we
started ignoring SIGPIPE in 8bf4becf0c, this could in theory still be
racy as described in that commit (because we die() on write failures
before pumping the connection for any ERR packets).

In practice this should be OK for this case. The server will not
actually check reachability until it has received our whole group of
"want" lines. And since we have no objects in the repository, we won't
send any "have" lines, meaning we're always waiting to read the server
response.

Note also that this case cannot happen in the v2 protocol, since it
allows any available object to be requested. However, we don't have to
take any steps to protect against the upcoming GIT_TEST_PROTOCOL_VERSION
in our tests:

  - the tests in t5516 would already need to be skipped under v2, and
    that is covered by ab0c5f5096 (tests: always test fetch of
    unreachable with v0, 2019-02-25)

  - the tests in t5530 invoke upload-pack directly, which will continue
    to default to v0. Eventually we may have a test setting which uses
    v2 even for bare upload-pack calls, but we can't override it here
    until we know what the setting looks like.

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