in merge-pull-request-trigger-server/src/main/java/org/jetbrains/teamcity/MergePullRequestTrigger.java [118:120]
private void saveRevision(@NotNull CustomDataStorage storage, @NotNull VcsRootInstance root, @NotNull String vcsBranch, @NotNull String revision) {
storage.putValue("revision:" + root.getParentId() + ":" + vcsBranch, revision);
}