[Integrated] RFR: vcs: add support for Mercurial 5.3

[Erik Helin]

Changeset: c23c574f
Author:    Erik Helin <ehelin at openjdk.org>
Date:      2020-02-14 10:50:51 +0000
URL:       https://git.openjdk.java.net/skara/commit/c23c574f

vcs: add support for Mercurial 5.3

Reviewed-by: rwestberg

! vcs/src/main/resources/ext.py