am: be more helpful

Heya,

I run into this problem every now and then when I manually apply a
patch. I apply it, do some stuff, and then having forgotten I already
applied it, try to apply it again, and am confused as to why it won't
apply. Would it be difficult to teach 'git am' to see if the patch-id
of the patch that couldn't be applied has been applied already
(similar to what 'git cherry' does I guess?) and print a helpful
message saying "warning: patch already applied" when there is a
conflict trying to apply such an already-applied patch?

-- 
Cheers,

Sverre Rabbelier