repo mirror list: reject multiple --sort, filter on owner/repo

Two review points:

• --sort is single-column, but a scalar StringVar silently kept the last of repeated flags. Switch to StringArrayVar and reject len>1 in PreRunE with a clear "only one --sort is supported" error, resolving the single value into sortSpec.
• --repo filtered on the bare repo name while the REPO column and sort key use owner/repo, so a value copied from the table (e.g. acme/web) matched nothing. Filter on the owner/repo form on both the mirror and --show-available paths so it matches the row it came from.