exp push: default to `--rev HEAD`

[dberenbaum]

Let's make dvc exp push origin push all experiments derived from the HEAD revision by default. This is especially useful in CI, where it's common to want to push the latest experiment and dvc exp push origin --rev HEAD feels like it's too complicated.

See discussion in https://iterativeai.slack.com/archives/C01116VLT7D/p1686321601633079.

[shcheklein]

Also, dvc exp push origin --rev HEAD seems to be not working on CI. I didn't manage to make it work the last time I tried (I didn't have enough time unfortunately). It comes down to the supported E2E scenarios and making sure that they are not broken / fairly simple to use.

[dberenbaum]

@shcheklein Added a separate ticket to discuss the more general problem.

[shcheklein]

@dberenbaum thanks a lot for moving forward this quickly! qq - do we plan to also make origin a default (similar to Git)?

[dberenbaum]

No, we discussed (and have discussed before) but decided not to for now for a few reasons:

1. Breaking change (because it's a positional arg, origin would be misread as an exp name) - CI/scripts will break, and Studio and VS Code need to coordinate this change
2. Potentially confusing behavior for what remote to track
3. Consistency with git (without upstream tracking branch, git will similarly fail)
4. Consistency with dvc exp list
5. Doesn't feel as painful as --rev HEAD
6. Bias towards explicit over implicit