RFR: 1934: Update ext.py to make test compatible with Mercurial6.4

[Erik Joelsson]

On Tue, 6 Jun 2023 21:58:48 GMT, Zhao Song <zsong at openjdk.org> wrote:

> Recently, I updated my mercurial  version to 6.4 and `RepositoryTests#testRemoteBranches` started to fail. 
> 
> After investigation, I found that `ui.expandpath` is deprecated and removed. To make our tests compatible with latest mercurial, we need to update method `ls_remote` in ext.py.
> 
> References:
> [1]:https://www.mail-archive.com/mercurial-devel@mercurial-scm.org/msg58072.html
> [2]:https://github.com/phacility/arcanist/commit/0fc22183e796fb8ac2e3a0a3f3f37aa964c6d7fa

Marked as reviewed by erikj (Lead).

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PR Review: https://git.openjdk.org/skara/pull/1526#pullrequestreview-1468163845