RFR: Make ext.py compatible with hg5 api

[Jorn Vernee via github.com]

The HG api changes in version 5 to drop the first 2 parameters to `mercurial.match.exact`, which breaks the extension script.

This PR detects whether we're running on hg 5, and if so, calls the function without the first 2 arguments instead.

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Commits:
 - 741ee3e3:	Make ext.py compatible with hg5 api

Pull request:
https://git.openjdk.java.net/skara/pull/67

Webrev:
https://webrevs.openjdk.java.net/skara/67/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/67.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/67/head:pull/67