revlog: introduce a `display_id` property
We currently using the "index file" to identify a revlog in error output. Since
we are about to make the "index file" location more volatile, we need something
better. We move to use the "radix", as it is close to what we currently use.
We could probably do better, as pointed out in the comment, however that would
be a quite detour from my current goal.
Differential Revision: https://phab.mercurial-scm.org/D10579
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