git-config: aliases with parameter expansion ($1 and such)

Hi, I've a question concerning git config and the alias things. I'd like to create an alias 'atag' that creates an annotated tag with the message being the same as the tag. Usually I create annotated tags like this:

$ git tag -a -m "mytag" mytag

(BTW: It's not documented, but I have the impression that the '-a' switch is implicitely added when the '-m' switch is present. Is that true?)

I'd like to have an alias like this:

[alias]
    atag = tag -a -m "$1"

So that I can simply call

$ git atag mytag

Is it possible to pass parameters to the alias definition? With the definition above I got a tag with message "$1".

Thanks, Dirk