Allow optional updating of symbolic references when fetching about gitoxide

[Byron]

Thanks for the hint, I will definitely take a look soonish!

If I understood git correctly, it really tries hard not to touch HEAD, and maybe the current gitoxide implementation is too broad to completely prohibit it. This might be a bug and I will see if that can just be enabled (except for setting HEAD), or if that should be configurable instead.

[Jake-Shadle]

Yup I think those two constraints make total sense as they (probably) won't affect the use case of updating a bare repository since those tend to be managed via code rather than manually by a user, so this change would effectively give the same benefits as fetch + checkout but for bare repositories.

[Byron]

This will be fixed with the next release, I will post here when that happens. Figuring this out was quite some work, but as always, that is rewarded. Now I think the logic is finally sound and more tested, too, which should allow gix to mirror repos pretty nicely.

I added a third rule to the above, repeated here for completeness:

---

The new logic will happily update any refs, symbolic or not, if…

• fast-forward rules aren't violated
• the target ref is not currently checked out
• existing refs would not become 'unborn', i.e. point to a reference that doesn't exist and won't be created due to ref-specs

---

[Byron]

This issue should be resolved with the latest release, but please let me know if anything is missing. Thanks you.

[Byron]

This will be improved with the next release, I will post here when that happens and it should be today or tomorrow - I am working on it.

It turns out that I definitely took a shortcut by simply not handling symbolic refs, which do complicate things quite a bit, but since some time passed I will it's a good time to tackle.
When using gix fetch I also noticed that it would refuse to do anything with origin/HEAD and thought that was unnecessary.

The new logic will happily update any refs, symbolic or not, if…

• fast-forward rules aren't violated
• if the target ref is not currently checked out

That should really be all that matters.