in merge-pull-request-trigger-server/src/main/java/org/jetbrains/teamcity/MergePullRequestTrigger.java [124:126]
private String readSavedRevision(@NotNull CustomDataStorage storage, @NotNull VcsRootInstance root, @NotNull String vcsBranch) {
return storage.getValue("revision:" + root.getParentId() + ":" + vcsBranch);
}