A user shouldn't perform a commit if any of the following are true:
* The repository state has changed since the last rescan.
* There are no files updated in the index to commit.
* There are unmerged stages still in the index.
* The commit message has not been provided.
* The pre-commit hook is executable and declined.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>