mergestate: add `allextras()` to get all extras

Authored by pulkit.


mergestate: add allextras() to get all extras

extras() can only be used for getting extra for a file. However at
couple of places in code, we wanted to iterate over all the extras stored with
the mergestate and they were accessing the private _stateextras.

We add a new function for this.

Differential Revision: https://phab.mercurial-scm.org/D9190