git pull fails

I'm trying to update from 2.7 to 2.8

when calling git pull I get this error:

[root@vmi10324 public_html]# git pull

Updating 2aa6dd4..cfab858

error: Your local changes to the following files would be overwritten by merge:

        admin/auth_config.php

.....

and the pull is aborted. 

What am I missing ?

Thanks ,

Dani