Add --skipmissing flag

This skips files that do not have a matching file in the other
directory.

vimtabdiff.py

@@ -32,6 +32,9 @@ def parse_args() -> argparse.Namespace:
     parser.add_argument(
         "--onlydiffs", help="only open files where there is a diff", action="store_true"
     )
+    parser.add_argument(
+        "--skipmissing", help="skip non-existent files", action="store_true"
+    )
     return parser.parse_args()
 
 
@@ -87,9 +90,10 @@ def main() -> None:
             aPath = a.resolve() if a else os.devnull
             bPath = b.resolve() if b else os.devnull
             if (
-                args.onlydiffs
+                (args.skipmissing and (not a or not b)) or
+                (args.onlydiffs
                     and a and b
-                and open(aPath, mode="rb").read() == open(bPath, mode="rb").read()
+                    and open(aPath, mode="rb").read() == open(bPath, mode="rb").read())
             ):
                 continue
             print(f"tabedit {aPath} | vsp {bPath}", file=vimCmdFile)