Added --keep-all, as an alias for '--keep-last <inf>', as an option…

… for borg-prune. Fixes borgbackup#6656

src/borg/archiver/prune_cmd.py

@@ -79,7 +79,7 @@ def do_prune(self, args, repository, manifest):
             self.print_error(
                 'At least one of the "keep-within", "keep-last", '
                 '"keep-secondly", "keep-minutely", "keep-hourly", "keep-daily", '
-                '"keep-weekly", "keep-monthly" or "keep-yearly" settings must be specified.'
+                '"keep-weekly", "keep-monthly", "keep-yearly" or "keep-all" settings must be specified.'
             )
             return self.exit_code
         checkpoint_re = r"\.checkpoint(\.\d+)?"
@@ -269,6 +269,13 @@ def build_parser_prune(self, subparsers, common_parser, mid_common_parser):
             default=0,
             help="number of secondly archives to keep",
         )
+        subparser.add_argument(
+            "--keep-all",
+            dest="secondly",
+            action="store_const",
+            const=float('inf'),
+            help="keep all archives (alias of --keep-last=<infinite>)"
+        )
         subparser.add_argument(
             "--keep-minutely", dest="minutely", type=int, default=0, help="number of minutely archives to keep"
         )