spec: Delete .git after applying patches

spec: Delete .git after applying patches

I'm hitting this little annoyance in fedora's package repo:

$ fedpkg prep
Downloading libvirt-1.2.20.tar.gz
...
+ /usr/bin/gzip -dc /home/crobinso/src/fedora/libvirt/libvirt-1.2.20.tar.gz
$ git clean -xdf
Removing libvirt-1.2.20.tar.gz
Skipping repository libvirt-1.2.20/

We git-ify the libvirt directory as part of applying patches in the spec
file, but 'git clean' will ignore subfolders that appear to be standalone
git repos.

Let's just delete the .git directory after we're done with it.

(cherry picked from commit 62ff210e5d6542bda3f25ed5e96c5088a840ce1b)