thanks for all the help massaging this diff

patch by Mikhail

The old code did not work correctly if only a single object Id was to be sent to got-read-pack. Make got-read-pack error out if the list of commits for object enumeration is empty to catch this problem if it occurs again. Found by the send_basic test, which was failing with GOT_TEST_PACK=1 ok tracey

This allows the main process to avoid looping over all object IDs again in case the pack file used for enumeration is complete. ok op@

in this case they're luckily the same; got_imsg_object_idlist and got_imsg_reused_delta only have one size_t long field. ok stsp@

use correct error code and ok stsp@

Simplifies the reading of the author/committer string and makes also the code shorter. ok stsp@

recv_imsg_error already checks the size of the message, and it does so more precisely than the removed code. ok stsp@

ints have the advantage that their size is more likely to be the same across the various architecture supported by OpenBSD, thus introducing less possible differences. INT_MAX is still (at least) a few order of magnitudes higher than the patches we dealt with (even abnormal ones.) suggested by stsp@

previous commit looked at some pretty zeroes returned from calloc instead of the actual numbers received.

under normal circumstances got-read-patch can't send negative line offsets; that's not an excuse not to guard the main process from those as well. This makes sure we bail out early instead of trying to apply the diff.