I'm following the git development of xf86-video-ati.   When I try to update via:

git pull --rebase 

I get:

From git://anongit.freedesktop.org/git/xorg/driver/xf86-video-ati
   5c11264..4f9d171  master     -> origin/master
You asked me to pull without telling me which branch you
want to merge with, and 'branch..merge' in
your configuration file does not tell me either.  Please
name which branch you want to merge on the command line and
try again (e.g. 'git pull <repository> <refspec>').
See git-pull(1) for details on the refspec.

If you often merge with the same branch, you may want to
configure the following variables in your configuration
file:

    branch..remote = <nickname>
    branch..merge = <remote-ref>
    remote.<nickname>.url = <url>
    remote.<nickname>.fetch = <refspec>

Trying 

git pull origin master 
or 
git reset --hard origin/master
works, until the next time.

None of the other Xorg modules seem to have this problem.