Add warning when there are changes in the index and using -a with git commit

Hi,

Many times I had the bad reflex of doing a git commit -as -m "blah
blah" when I was willing to commit only things I had staged in the
index.

It could be useful to add a warning and a prompt when using the -a
option with changes staged in the index to avoid errors.

Regards.

-- 
Sylvain