gerrit should include merge commit sha1 in change-merged events

[James E. Blair]

The change-merged event only includes information about the proposed change that was merged, but if a merge commit was generated for that commit, then it is not included. Typically actions that we would want to take in jenkins would be on the tree with the merge-commit as tip rather than the original commit (which may be based on something other than the current tip). So gerrit should be modified to include that SHA1, and the gerrit trigger plugin should be modified to (optionally?) use it if present.

See: gerrit-server/src/main/java/com/google/gerrit/server/git/MergeOp.java

[James E. Blair (corvus)]

Note that if multiple changes with dependencies get merged at once (because their ancestor is merged) there will be one merge commit used for all the changes. Example:

* e501593 Merge changes Ic9f0d336,Ia4a7402a,I8a62490c
|\
| * c83efb1 test c
| * d3ab4ca test b
| * 57c10a0 test a
* | 3c194f3 test merge
|/
* 9c2d83c common head

(test a, test b, and test c (all based on "common head" were merged into "test merge" via the gerrit generated merge commit on top))

In that case we may be using the same merge commit sha1 for multiple events. It's probably the correct thing to do, however, we'll want to know that when writing jenkins jobs. We may end up making the same tarball twice (however, it should be the _same_ tarball, for whatever that's worth).

[Monty Taylor (mordred)]

As part of this, having the ability to have jenkins write the merge SHA back into the gerrit review comment "change merged" ... and also having gerrit add a note about the Change-ID into the commit message for the merge commit.

[George Young (georgey21)]

Added request for Gerrit event extension here: http://code.google.com/p/gerrit/issues/detail?id=2785

[George Young (georgey21)]

and here: https://issues.jenkins-ci.org/browse/JENKINS-23873