Xgit.Repository.put_ref/4: Allow creating a sym ref to a non-existent ref.

[scouten]

Changes in This Pull Request

This is needed for the proper implementation of creating a symbolic ref.

git symbolic-ref (name) (target ref) does not fail if (target ref) does not yet exist. To wit (in an otherwise empty repo):

$ git symbolic-ref HEAD refs/heads/nope
$ git status
On branch nope

Initial commit

nothing to commit (create/copy files and use "git add" to track)

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes.
• All cases where a literal value is returned use the cover macro to force code coverage.
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a