Figure out {commit} for Travis CI {build_commit} for merged PRs

[yarikoptic]

Travis provides

(Pdb) rich.inspect(build['commit'])
│ │ {                                                                            │ │
│ │ │   '@type': 'commit',                                                       │ │
│ │ │   '@representation': 'minimal',                                            │ │
│ │ │   'id': 556084708,                                                         │ │
│ │ │   'sha': '03f35ebbd696894fe61c2fff46217bff60019db8',                       │ │
│ │ │   'ref': 'refs/pull/68/merge',                                             │ │
│ │ │   'message': 'Fill in `{commit}` for Travis PR builds by querying GitHub', │ │
│ │ │   'compare_url': 'https://github.com/con/tinuous/pull/68',                 │ │
│ │ │   'committed_at': '2021-06-01T16:23:50Z'                                   │ │
│ │ }                                            

and to deduce the HEAD commit we have

            pr = self.ghrepo.get_pull(build["pull_request_number"])
            if pr.merge_commit_sha == build["commit"]["sha"]:
                return pr.head.sha

but for a now merged merged PR #68 we have now b7ab0f3 in the master as pr.merge_commit_sha not the (build["commit"]["sha"]) 03f35eb as it was the merge commit whenever PR was not yet merged.

There is a ticket @jwodder filed with Travis to have HEAD {commit} to be also be part of the Travis API record, but it is still unresolved. So the question is either there is another way , and it seems to be via query to github about parents of the PR merge head commit as known to Travis:

$> curl --silent https://api.github.com/repos/con/tinuous/commits/03f35ebbd696894fe61c2fff46217bff60019db8 | jq '.parents'
[
  {
    "sha": "52c92e0b0cc683e53ebde25f7ec32cd98434e2ba",
    "url": "https://api.github.com/repos/con/tinuous/commits/52c92e0b0cc683e53ebde25f7ec32cd98434e2ba",
    "html_url": "https://github.com/con/tinuous/commit/52c92e0b0cc683e53ebde25f7ec32cd98434e2ba"
  },
  {
    "sha": "c30b87e74a47cd360306f0674f793b14c79843da",
    "url": "https://api.github.com/repos/con/tinuous/commits/c30b87e74a47cd360306f0674f793b14c79843da",
    "html_url": "https://github.com/con/tinuous/commit/c30b87e74a47cd360306f0674f793b14c79843da"
  }
]

so I guess we could take those and if any matches the pr.head.sha - we have unambigous winner. Otherwise -- could we rely on the fact that first commit would be from the target branch or do some other check or it would not even be needed?