Writes using branch reference syntax (ie. `<db>/<branch>`) should fail

[zachmu]

Dolt's transaction logic assumes all changes are on a single branch, the currently checked out one for the session. But there's a loophole:

update `mydb/otherbranch`.mytable set foo = baz;

This behavior is undefined and won't work correctly on commit. These databases should be read-only in this context, and these writes should fail the same way they do on read-only databases.

[jycor]

Maybe related? https://github.com/dolthub/dolt/issues/4925

[timsehn]

So this is fixed and replace by #5314