upload-pack: allow stateless client EOF just prior to haves

During stateless packfile negotiation where a depth is given, stateless RPC clients (e.g. git-remote-curl) will send multiple upload-pack requests with the first containing only the wants/shallows/deepens/filters and the subsequent containing haves/done. When upload-pack handles such requests, entering get_common_commits without checking whether the client has hung up can result in unexpected EOF during the negotiation loop and a die() with message "fatal: the remote end hung up unexpectedly". Real world effects include: - A client speaking to git-http-backend via a server that doesn't check the exit codes of CGIs (e.g. mod_cgi) doesn't know and doesn't care about the fatal. It continues to process the response body as normal. - A client speaking to a server that does check the exit code and returns an errant HTTP status as a result will fail with the message "error: RPC failed; HTTP 500 curl 22 The requested URL returned error: 500." - Admins running servers that surface the failure must workaround it by patching code that handles execution of git-http-backend to ignore exit codes or take other heuristic approaches. - Admins may have to deal with "hung up unexpectedly" log spam related to the failures even in cases where the exit code isn't surfaced as an HTTP server-side error status. To avoid these EOF related fatals, have upload-pack gently peek for an EOF between the sending of shallow/unshallow lines (followed by flush) and the reading of client haves. If the client has hung up at this point, exit normally. Signed-off-by: Daniel Duvall <dan@mutual.io> Signed-off-by: Junio C Hamano <gitster@pobox.com>