git-am: Please don't be in "some state" when it fails early

If git am fails early, before even trying to apply the first patch, it
would be great if am did not leave some state that has to be manually
cleaned up (git am --abort). Even if I have a small 'AM' marker in my
prompt, it is often missed. And why write state files to .git/ at all,
if all it did was to tell me that the index is dirty?

This fixes an inconvenience, and I don't have to wonder (look up)
every time if git am --abort will delete all the WIP I had in the way
or not.

Ulrik