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

t/t5400-send-pack.sh

@@ -288,7 +288,10 @@ test_expect_success 'receive-pack de-dupes .have lines' '
 	$shared .have
 	EOF
 
-	GIT_TRACE_PACKET=$(pwd)/trace git push fork HEAD:foo &&
+	GIT_TRACE_PACKET=$(pwd)/trace \
+	    git push \
+		--receive-pack="unset GIT_TRACE_PACKET; git-receive-pack" \
+		fork HEAD:foo &&
 	extract_ref_advertisement <trace >refs &&
 	test_cmp expect refs
 '