Merge branch 'bc/receive-pack-stdout-protection' into maint-1.7.11

When "git push" triggered the automatic gc on the receiving end, a message from "git prune" that said it was removing cruft leaked to the standard output, breaking the communication protocol. * bc/receive-pack-stdout-protection: receive-pack: do not leak output from auto-gc to standard output t/t5400: demonstrate breakage caused by informational message from prune