RFR: Improve merge bot commit message

[Jorn Vernee]

The current commit message the merge bot uses just says

> Merge

I'd like it if the message contained the branches involved as well, to be a little more informative.

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

Commits:
 - 610082af: Improve merge bot commit message

Changes: https://git.openjdk.java.net/skara/pull/400/files
 Webrev: https://webrevs.openjdk.java.net/skara/400/webrev.00
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/400.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/400/head:pull/400

PR: https://git.openjdk.java.net/skara/pull/400