The underlying problem in Git is that commit-msg hooks are not called for merge commits without conflicts.
However, the prepare-commit-msg is called for (any) merge commits. So what I currently do to get Gerrit's Change-Id also added to merge commits without conflicts is to use a prepare-commit-msghook like this:
#!/bin/sh
if [ "$2" = "merge" -a -f .git/MERGE_MSG ]; then
# Explicitly call Gerrit's commit-msg hook for merge commits.
.git/hooks/commit-msg "$1"
fi
The check for .git/MERGE_MSG ensures that commit-msg will not be called if amending a merge commit, because in that case that hook is called directly by Git anyway. Note that for merge commits with conflicts this approach will result in commit-msg being called twice, once as part of this prepare-commit-msg hook and once by Git when it calls commit-msg after committing the conflicts resolution, but that does not cause any problems as Gerrit's commit-msg hook checks whether the Change-Id has already been added and does not add it again if that's the case.
If you're now asking your self why Gerrit does not simply use only a prepare-commit-msg hook to add the Change-Id, I guess that just to prevent the user from accidentally deleting the Change-Idfrom the commit message. Adding it as part of a commit-msg hook after the editor has closed is just safer.
http://stackoverflow.com/questions/19917025/why-does-gerrit-not-include-the-change-id-into-merge-commits