contrib/subtree: fix broken &&-chains and revealed test error

contrib/subtree: fix broken &&-chains and revealed test error

This fixes two instances where a &&-chain was broken in the subtree
tests and fixes a test error that was revealed because of this.

Many tests in t7900-subtree.sh make a commit and then use 'undo' to
reset the state for the next test. In the 'check hash of split' test,
an 'undo' was being invoked after a 'subtree split' even though the
particular invocation of 'subtree split' did not actually make a commit.
The subsequent check_equal was failing, but this failure was masked by
that broken &&-chain.

Removing this undo causes the failing check_equal to succeed but breaks
the a check_equal later on in the same test.

It turns out that an earlier test ('check if --message for merge works
with squash too') makes a commit but doesn't 'undo' to the state
expected by the remaining tests. None of the intervening tests cared
enough about the state of the test repo to fail and the spurious 'undo'
in 'check hash of split' restored the expected state for any remaining
test that might care.

Adding the missing 'undo' to 'check if --message for merge works
with squash too' and removing the spurious one from 'check hash of
split' fixes all tests once the &&-chains are completed.

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>