Minor prompt fixes.

* magit.el (magit-process-sentinel): Double space to end sentence.
  (magit-merge-abort): Add space at end of `yes-or-no-p' prompt.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #971 from ruediger/minor-message-fix

Minor prompt fixes.