Add option to diff command to get the meta changes in all layers

[volaya]

Now the meta changes for a given layer can be obtained using

kart diff refa...refb datasetname:meta -ojson

But there is no way of showing exisiting meta changes for all layers modified between the selected commits.

A bit of context for this: In the plugin, we are trying to detect whether there are meta changes or no before opening elements such as the diff viewer, which for now assume that the schema doesnt change

[craigds]

Obvious way to implement this would be:

kart diff refa...refb *:meta -ojson

Since we forbid the * character in dataset names, this results in unambiguous syntax.

The only obvious problem is that * will be expanded by shells to a filename, if a file matching that pattern exists in the current directory. That seems pretty unlikely though and it can always be quoted, so probably not a big deal.

• If we do this we should make it work for *:feature too
• Also *:<featureID> - this seems easy enough to make work, and means for a single-layer repo, the user doesn't have to use the actual dataset name, which they may not remember.