RFR: mlbridge: configure webrev ref explicitly

[Erik Helin]

Hi all,

please review this small patch makes the configuration for the ref used for storing webrevs explicit. Now that mlbridge
supports two different kinds of webrevs, the ref applies to both repositories and must therefore be configured
separately.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - mlbridge: configure webrev ref explicitly

Changes: https://git.openjdk.java.net/skara/pull/721/files
 Webrev: https://webrevs.openjdk.java.net/skara/721/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/721.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/721/head:pull/721

PR: https://git.openjdk.java.net/skara/pull/721