git-serve: Use `pkgbase` for errors about invalid names

`repo` is not defined at the point it's used, so you would get errors
like

> Traceback (most recent call last):
>   File "/srv/http/vhosts/aur-dev.archlinux.org/public/git-interface/git-serve.py", line 138, in <module>
>     die('%s: invalid repository name: %s' % (action, repo))
> NameError: name 'repo' is not defined
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

git-interface/git-serve.py

@@ -135,7 +135,7 @@ if action == 'git-upload-pack' or action == 'git-receive-pack':
         die('%s: invalid path: %s' % (action, path))
     pkgbase = path[1:-4]
     if not re.match(repo_regex, pkgbase):
-        die('%s: invalid repository name: %s' % (action, repo))
+        die('%s: invalid repository name: %s' % (action, pkgbase))
 
     if not pkgbase_exists(pkgbase):
         create_pkgbase(pkgbase, user)