checkcommit: Don't check out original commit

New versions of Git change the output of git branch (which we shouldn't
have depended on in the first place), so the script breaks. Looking at
the git-prompt.sh code, getting back to the original state is really
hard, so just don't do it.
Switch to building with jUnit 4
Allow static/star imports from org.junit.Assert
Import from org.junit in ConfigurableAccountManager
Don't allow static imports from jUnit3 Assert
Run AccountManagerTest with jUnit4
Run MessageBankTest with jUnit4
Run MailMessageTest with jUnit4
Run MailHeaderFilter with jUnit 4
Run FreenetURI with jUnit 4
Run EmailAddress with jUnit 4
Run OwnIdentity with jUnit 4
Run IdentityMatcher with jUnit 4
Run IMAP tests with jUnit4
Run DateStringFactoryTest with jUnit 4