Override new abstract methods in RepositoryTestCase.MockSystemReader

org.eclipse.jgit.util.SystemReader was extened with two abstract
methods getTimezone(long) and getCurrentTime(). Implement them and
always return 0.

Signed-off-by: Mykola Nikishov <mn@mn.com.ua>

nbgit 0.3

New features:

 - Initial support for cloning; Only git:// protocol is supported for
   now. (issue 48)
 - Graph visualization of repository history. (issue 1)
 - Check email addresses entered though the properties UI. (issue 4)

Bug fixes:

 - Track changes to and refresh internally cached .gitignore files.
   (issue 3)
 - Gracefully handle repositories containing submodules. (issue 23,
   issue 33, issue 34)
 - Fix revert when using folder/project selection. (issue 51)

nbgit 0.2

New features:

 - Optionally strip spaces from commit messages. (issue 8)
 - Support for .gitignore files. (issue 3)

Bug fixes:

 - Make code compatible with Java 1.5. (issue 22 and issue 21)
 - Update the status cache after creating a repository. (issue 24)
 - Bundle version utilities with the nbm file. (issue 19)

nbgit 0.1