RFR: 1283: Test failures with Mercurial 6.0

[Erik Joelsson]

Adjust ext.py to handle Mercurial versions 6.0+.

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Commit messages:
 - SKARA-1283

Changes: https://git.openjdk.java.net/skara/pull/1261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1261&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1283
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1261.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1261/head:pull/1261

PR: https://git.openjdk.java.net/skara/pull/1261