t6022 (renaming merge): chain test commands with &&

t6022 (renaming merge): chain test commands with &&

Using 'return' in an attempt to end a test assertion can have
unpredictable results (probably escaping from test_run_ and breaking
its bookkeeping).  Redo the control flow using helpers like
test_expect_code and git diff --exit-code, so each test assertion can
follow the usual form

	command that should succeed &&
	command that should succeed &&
	command that should succeed &&
	...

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>