Manually trigger build from old tag / commit

[native-api]

It’s possible to manually trigger a build via API but only for valid remote refs (specifically, for what can be passed to git clone --branch).

If the commit you are interested in doesn’t qualify, you can always get around that by creating a branch or a tag at it (then a build would be created automatically since that’s a new change).

Trigger a specific commit via the Travis API

Yes, it’s only possible to specify branch names (more specifically, valid remote refs). I’ve tried supplying a SHA-1 instead as branch and a build was created. But the supplied name is being passed to git clone --branch and the cloning thus failed. Nothing prevents you from creating a topic branch at any commit though. (See also https://stackoverflow.com/questions/14370157/git-fetch-a-specific-commit-by-hash for what valid input is in Git terms.)