filter-repo: fix an incorrect exit

While most users of filter-repo will just use it as a tool and
RepoFilter.run() is the final function, filter-repo can be used as a
library with additional work being done after calling that function.
So, simply return from that function when it is done rather than calling
sys.exit.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -3098,7 +3098,7 @@ class RepoFilter(object):
       print("NOTE: Not running fast-import or cleaning up; --dry-run passed.")
       print("      Requested filtering can be seen {}".format(orig_str))
       print("        " + self._fe_filt)
-      sys.exit(0)
+      return
 
     target_working_dir = self._args.target or '.'
     if self._input: