Fix rollback on only one migration

[moritz-h]

Fix #1059

I think error comes with this commit: bba44b3
If only only migration is left the first migration is selected as target instead of 0. So nothing is done.

I found a test case for this, but don't know why the test does not work:
https://github.com/cakephp/phinx/blob/v0.8.0/tests/Phinx/Migration/ManagerTest.php#L712

[rquadling]

A negative assertion, only tests what shouldn't happen, not what does happen. Hmmm.

[moritz-h]

I played with the assertions. So ether add to assert not contains No migrations to rollback
or to contains some of this:

== 20120111235330 TestMigration: reverting
 == 20120111235330 TestMigration: reverted 0.0000s

[rquadling]

Hi.

If you can make the test assert positively (i.e. that when there is 1 or 0 migrations, the result is appropriate, then I can merge this.

[moritz-h]

I added both now. Positive assertion for the "reverting" and "reverted" status output.
Also the negative assertion to not contain "no migrations", so for what ever could go wrong, the test could fail on this.

[rquadling]

Thank you.

[glensc]

@moritz-h (since you are frequent contributor): do not link to branches, use git tag (or commit hash) to link to sourcecode, because the line (or even file) referenced gets out of sync, so the links in issue make no sense in the future.

[moritz-h]

@glensc ok, I try to do this in future.
edit: (done for the above link)