Have a look at Source Stamp Properties There in changes you have access to these attributes.
So in a custom buildStep you could do something like:
def start(self):
branch = self.getProperty('branch')
revision = self.getProperty('revision')
repository = self.getProperty('repository')
changes = self.getProperty("changes") or [] # returns None if there are no changes
for ch in changes:
changed_by = ch.who
changed_at = ch.when
comments = ch.comments
changed_files = ch.files
#save to file in master
#call super start