magit-status: bug fix when initializing.

Because magit-init returns nil even when a repo is created, the code in
magit-status that brings up the status buffer wasn't executed.

Thanks to kyleam for finding the misgiving code.

Fix #1562 on master
Merge pull request #1570 from vanicat/t/status-init

magit-status: bug fix when initializing.
AUTHORS.md: update list of contributors