Adds new extension point to validate a branch tip update when a change is being
submitted by submit strategy. As submit strategies may generate new commits
(e.g. Cherry Pick), this listener allows validation of resulting new commit
before branch is updated, potentially aborting the submission.
Before, only MergeValidationListener.onPreMerge was available to
validate changes being submitted. However, onPreMerge was called
before submit strategies were run, thus commits generated by submit
strategies were previously impossible to validate.
Now, OnSubmitValidationListener.preBranchUpdate can be used
to abort submission before any refs are updated, even if there are
several changes submitted together.
Change-Id: I2ab75c7c38eb8ed0c010507b49f40e1b5cebad3b
da2034f1a3