Support SHA1 in source of RefSpec for fetch

[bgardiner]

If uploadpack.allowReachableSHA1InWant is True on the git server, a fetch request can specify a SHA1 in the want line. I do not believe Fetch currently supports specifying a SHA1 as the source refspec. When attempting this, there is no fetch performed and I receive a NoErrAlreadyUpToDate. For example:

refspec := config.RefSpec(fmt.Sprintf("%v:%v", reference.Hash().String(), "refs/remotes/name/branch"))
	if err = r.FetchContext(ctx, &git.FetchOptions{
		RemoteName: "name",
		RefSpecs:   []config.RefSpec{refspec},
		Depth:      1,
		Auth:       auth,
	}); err != nil {
		return errors.Wrap(err)

I'm using v4.0.0-rc15 (the latest release) of the package. Am I not specifying the RefSpec correctly or is this a limitation in the current implementation?

[mil]

+1 curious if this feature is already supported and just not documented; if not I think per-SHA fetch would be a great addition.

[mcuadros]

Currently is not supported, the refspec only works currently with references

[ivan-section-io]

Server side, I think this includes any option that allows non-ref fetches (SHA1 wants): allowTipSHA1InWant, allowReachableSHA1InWant, allowAnySHA1InWant https://git-scm.com/docs/git-config#git-config-uploadpackallowTipSHA1InWant
Client side, a fetch refspec: " is typically a ref, but it can also be a fully spelled hex object name." https://git-scm.com/docs/git-fetch#git-fetch-ltrefspecgt

Seems to also be behind issue #691