Merge branch 'jk/alternate-ref-optim'

A test allowed both "git push" and "git receive-pack" on the other end write their traces into the same file. This is OK on platforms that allows atomically appending to a file opened with O_APPEND, but on other platforms led to a mangled output, causing intermittent test failures. This has been fixed by disabling traces from "receive-pack" in the test. * jk/alternate-ref-optim: t5400: avoid concurrent writes into a trace file