Fix regression: can fetch branches and tags references without specifying commit hashes for private git repository used as context

[gcalmettes]

Fixes #1801

Description

This PR fixes a regression introduced in #1765 (PR released in v1.7.0) that led to the need of always specifying a commit hash even for tags or branches references, when a git source context was used.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

[gcalmettes]

Rescan needed from a repository manager (post CLA signature). Thanks !

[imjasonh]

LGTM, but would it be possible to add an integration test to ensure this behavior doesn't regress in a future change?

[gcalmettes]

Thanks @imjasonh for the review.

Regarding implementing some integration tests, this would mean adding a test involving the build from a private repository, so that the authentication is required. Right now, Kaniko GitHub repository is used in the integration tests to check the git context from a public repository.

I could definitely add some integration tests involving a local git server, the same way the use of a local registry is described in the Development guide (in this case, using a docker image of Gogs for example could be an option, as it is very easy to setup).
Another option would be to connect to a private registry (hosted on GitHub for example). I do not know if there is already a dedicated authenticated repository for such a purpose, but this can definitely be also made.

What do you think ?

[imjasonh]

Makes sense. If this causes more trouble in the future we can add a test then.

Can you rebase on master to make sure current tests pass? We've made a lot of changes since this PR was opened.

Thanks for this contribution, and for your patience!

[gcalmettes]

Thanks!
Rebase on master done ✔️

[pnbecker]

Thank you @imjasonh and @gcalmettes for working on this! The bug was quite annoying for us.