bfg-ish: add some sanity checks on the specified repo

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

contrib/filter-repo-demos/bfg-ish

@@ -174,6 +174,19 @@ class BFG_ish:
             help=("file path for Git repository to clean"))
 
     args = parser.parse_args()
+
+    # Sanity check on args.repo
+    if not os.path.isdir(args.repo):
+      raise SystemExit("Repo not found: {}".format(os.fsdecode(args.repo)))
+    dirname, basename = os.path.split(args.repo)
+    if not basename:
+      dirname, basename = os.path.split(dirname)
+    if not dirname:
+      dirname = b'.'
+    if basename == b".git":
+      raise SystemExit("For non-bare repos, please specify the toplevel directory ({}) for repo"
+                       .format(os.fsdecode(dirname)))
+
     return args
 
   def convert_replace_text(self, filename):