cloneRefName() does not support annotated git tags

[bburky]

cloneRefName() does not support annotated git tags and attempts to clone the annotated tag object's id as if it were a commit object. This causes the following error (with the tag's hash, not the commit):

unable to resolve commit object for 'da39a3ee5e6b4b0d3255bfef95601890afd80709': object not found

Currently Flux's cloneRefName() passes the ref to getRemoteHEAD() to resolve the ref to an object id, and then passes the id to cloneCommit() which only supports commit objects, not annotated tags.

A configuration like the following will trigger the error:

apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
  name: repository-name
spec:
  ref:
    name: refs/tags/an-annotated-git-tag

We accidentally stumbled across this when a new developer made the tag and they used an annotated tag not expecting there to be any difference at all for Flux. We had to a little bit of debugging to figure out that an annotated tag caused the issue. I think most users would expect any ref to work with name:.

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See #539 for discussion and #539 (comment) for a proposed solution