Use --prune with git fetch

Fixes this failure:

From git://sourceware.org/git/glibc
 + b570759fc5...d8972c5030 hjl/cet/pr21598            -> hjl/cet/pr21598  (forced update)
 + 57f2b07c55...2a355dbc47 hjl/cet/property           -> hjl/cet/property  (forced update)
error: cannot lock ref 'refs/heads/hjl/pr21913/master': 'refs/heads/hjl/pr21913' exists; cannot create 'refs/heads/hjl/pr21913/master'
 ! [new branch]            hjl/pr21913/master         -> hjl/pr21913/master  (unable to update local ref)
   cd65836b5f..36ada5f681  master                     -> master
 * [new branch]            origin/release/2.25/master -> origin/release/2.25/master
   05dcf1ea88..5b3f978ccf  release/2.25/master        -> release/2.25/master

TarSCM/scm/git.py

@@ -115,7 +115,7 @@ def update_cache(self):
             interactive=sys.stdout.isatty()
         )
         self.helpers.safe_run(
-            ['git', 'fetch'],
+            ['git', 'fetch', '--prune'],
             cwd=self.clone_dir,
             interactive=sys.stdout.isatty()
         )