prune: Handle --max-repack-size=0 as expected

Previously the flag was ignored and `--max-repack-size=1` had to be
used.

changelog/unreleased/pull-3591

@@ -0,0 +1,8 @@
+Bugfix: Fix handling of `prune --max-repack-size=0`
+
+Restic ignored the `--max-repack-size` option when passing a value of 0. This
+has been fixed.
+
+As a workaround, `--max-repack-size=1` can be used with older versions of restic.
+
+https://github.com/restic/restic/pull/3591