Current GitPoller misses merge commits

[jasugun]

Hi, the current git poller _git_commit_files lists no files for merge commits (with no conflicts)
This makes the poller totally miss these merge commits.
When merging diffrent branches, it has the potential to trigger scedulers based off wrong commits from other branches.

Is this on purpose or can I make a PR with a fix (I think adding -m to the git command would fix the issue) here?
https://github.com/buildbot/buildbot/blob/master/master/buildbot/changes/gitpoller.py#L354