Check for staged files before running the prompt. Running the `git-cz` command with no files staged
reports "No files added to staging". Preserve the functionality of `git -a` (--all) flag - `git-cz
-a` command with no files added to staging area, adds all files to staging and opens the prompt (no
error thrown).

"fix commitizen#785"

Extend upon the existing commit unit tests. Assert the error is thrown when running commitizen
commit command with no files add to staging.

"re commitizen#585"

Extend upon the existing staging unit tests. Assert that the files are added to staging area when
running commitizen commit command with -a (--all) flag, prior to spawning the prompt.

"re commitizen#785"