Merge branch 'jk/alternate-ref-optim' into maint

Merge branch 'jk/alternate-ref-optim' into maint

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