Expand filenames for --annotate before launching editor

I have GIT_EDITOR=vim, and the vim version I use won't expand "*" and
will simply try to create a new file named "/tmp/.../*". Expand the
filename list before invoking $GIT_EDITOR so that doesn't happen.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>