RFR: vcs: add support for Mercurial 5.3

[Erik Helin]

Hi all,

please review this patch that adds support for Mercurial 5.3

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Commits:
 - c60e44aa: vcs: add support for Mercurial 5.3

Changes: https://git.openjdk.java.net/skara/pull/438/files
 Webrev: https://webrevs.openjdk.java.net/skara/438/webrev.00
  Stats: 13 lines in 1 file changed: 6 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/438.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/438/head:pull/438

PR: https://git.openjdk.java.net/skara/pull/438