demonstrate git-commit --dry-run exit code behaviour

demonstrate git-commit --dry-run exit code behaviour

In particular, show that --short and --porcelain, while implying
--dry-run, do not return the same exit code as --dry-run. This is due to
the wt_status.commitable flag being set only when a long status is
requested.

No fix is provided here; with [1], it should be trivial to fix though -
just a matter of calling wt_status_mark_commitable().

[1] http://article.gmane.org/gmane.comp.version-control.git/242489

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>