git-gui: Verify we should actually perform a commit when asked to do so.

git-gui: Verify we should actually perform a commit when asked to do so.

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>