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

[Zhao Song]

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

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Commit messages:
 - SKARA-1934

Changes: https://git.openjdk.org/skara/pull/1526/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1526&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1934
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1526.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1526/head:pull/1526

PR: https://git.openjdk.org/skara/pull/1526