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.
fix some strings
AUTHORS.md: update list of contributors
Merge branch 'master' into next

Conflicts:
	magit.el

Discard the backport.
magit-diff-visit-file: use magit-find-file when appropriate

When visiting a committed change and the commit isn't the HEAD commit
then visit the file using `magit-find-file-noselect' instead of visiting
the working tree.

This partially implements #1566.
magit-{,branch-or-}commit-at-point: in rev buffers return shown rev

In `magit-revision-mode' buffers `magit-branch-or-commit-at-point' and
`magit-commit-at-point' now return the commit shown in that buffer.
magit-diff-dwim: bugfix