t5573, t7612: clean up after unexpected success of 'pull' and 'merge'

t5573, t7612: clean up after unexpected success of 'pull' and 'merge'

The previous steps added test_when_finished to tests that run 'git
pull' or 'git merge' with expectation of success, so that the test
after them can start from a known state even when their 'git pull'
invocation unexpectedly fails.  However, tests that run 'git pull'
or 'git merge' expecting it not to succeed forgot to protect later
tests the same way---if they unexpectedly succeed, the test after
them would start from an unexpected state.

Reset and checkout the initial commit after all these tests, whether
they expect their invocations to succeed or fail.

Signed-off-by: Junio C Hamano <gitster@pobox.com>