Alias for all indexes

[monreal]

On some workflows I want to say pin/unpin all indexes. Right now I need to look up how many indexes there are and issue commands individually. It would be great to have a special index like "all" to apply to all indexes.

[ericcurtin]

This is a nice accessible code change, the corresponding code is here:

https://github.com/ostreedev/ostree/blob/main/src/ostree/ot-admin-builtin-pin.c