Fix flaky t5504 (in test case 9 - push with transfer.fsckobjects)

[dscho]

This seems to hit the linux-gcc job the most, maybe in the part where it runs the test suite with all kinds of GIT_TEST_* variables set to non-default settings.

See e.g. Attempt #1 of this linux-gcc run:

expecting success: 
	rm -rf dst &&
	git init dst &&
	(
		cd dst &&
		git config transfer.fsckobjects true
	) &&
	test_must_fail git push --porcelain dst master:refs/heads/test >act &&
	test_cmp exp act

+ rm -rf dst
+ git init dst
Initialized empty Git repository in /home/vsts/work/1/s/t/trash directory.t5504-fetch-receive-strict/dst/.git/
+ cd dst
+ git config transfer.fsckobjects true
+ test_must_fail git push --porcelain dst master:refs/heads/test
+ _test_ok=
+ git push --porcelain dst master:refs/heads/test
remote: fatal: object of unexpected type        
error: remote unpack failed: unpack-objects abnormal exit
error: failed to push some refs to 'dst'
+ exit_code=1
+ test 1 -eq 0
+ test_match_signal 13 1
+ test 1 = 141
+ test 1 = 269
+ return 1
+ test 1 -gt 129
+ test 1 -eq 127
+ test 1 -eq 126
+ return 0
+ test_cmp exp act
+ diff -u exp act
--- exp	2019-06-03 20:37:40.928834316 +0000
+++ act	2019-06-03 20:37:41.064834272 +0000
@@ -1,2 +0,0 @@
-To dst
-!	refs/heads/master:refs/heads/test	[remote rejected] (unpacker error)
error: last command exited with $?=1
not ok 9 - push with transfer.fsckobjects
#	
#		rm -rf dst &&
#		git init dst &&
#		(
#			cd dst &&
#			git config transfer.fsckobjects true
#		) &&
#		test_must_fail git push --porcelain dst master:refs/heads/test >act &&
#		test_cmp exp act
#