Baseref is sometimes not found

[korthout]

I've seen this message a couple of times in situations where I did not expect it.

Like in camunda/camunda#8038 (comment)

Backport failed for stable/1.2, because 1 or more of the commits are not available.

Please cherry-pick the changes locally.
Note that rebase and squash merges are not supported at this time.
For more information see zeebe-io/backport-action#46.

On further inspection of the logs of the workflow run, I noticed that the baseref is not a known commit.

Working on label backport stable/1.1
Found target in label: stable/1.1
From https://github.com/camunda-cloud/zeebe
 * branch                  stable/1.1 -> FETCH_HEAD
 * [new branch]            stable/1.1 -> origin/stable/1.1
Start backport to backport-8038-to-stable/1.1
fatal: git cat-file: could not get object info
exitcode(6): Backport failed for `stable/1.1`, because 1 or more of the commits are not available.

This is very hidden, but exitcode 6 means that the cat-file check on the baseref failed. We should also improve error messages, but that's a different topic.

The baseref is not yet directly fetched, something that should've been done in #162.