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2f06c80bd0
support for shallow clones and fetches with `gitoxide`
This PR makes it possible to enable shallow clones and fetches for git dependencies and crate indices independently with the `-Zgitoxide=fetch,shallow_deps` and `-Zgitoxide=fetch,shallow_index` respectively.
### Tasks
* [x] setup the shallow option when fetching, differentiated by 'registry' and 'git-dependency'
* [x] validate registries are cloned shallowly *and* fetched shallowly
* [x] validate git-dependencies are cloned shallowly *and* fetched shallowly
* [x] a test to show what happens if a shallow index is opened with `git2` (*it can open it and fetch like normal, no issues*)
* [x] assure that `git2` can safely operate on a shallow clone - we unshallow it beforehand, both for registries and git dependencies
* [x] assure git-deps with revisions are handled correctly (they should just not be shallow, and they should unshallow themselves if they are)
* [x] make sure shallow index clones aren't seen by older cargo's
* [x] make sure shallow git dependency clones aren't seen by older cargo's
* [x] shallow.lock test and more test-suite runs with shallow clones enabled for everything
* [x] release new version of `gix` with full shallow support and use it here
* [x] check why `shallow` files remain after unshallowing. Should they not rather be deleted if empty? - Yes, `git` does so as well, implemented [with this commit](2cd5054b0a)
* ~~see if it can be avoided to ever unshallow an existing `-shallow` clone by using the right location from the start. If not, test that we can go `shallow->unshallow->shallow` without a hitch.~~ Cannot happen anymore as it can predict the final location perfectly.
* [x] `Cargo.lock` files don't prevent shallow clones
* [x] assure all other tests work with shallow cloning enabled (or fix the ones that don't with regression protection)
* [x] can the 'split-brain' issue be solved for good?
### Review Notes
* there is a chance of 'split brain' in git-dependencies as the logic for determining whether the clone/fetch is shallow is repeated in two places. This isn't the case for registries though.
### Notes
* I am highlighting that this is the `gitoxide` version of shallow clones as the `git2` version [might soon be available](https://github.com/libgit2/libgit2/pull/6396) as well. Having that would be good as it would ensure interoperability remains intact.
* Maybe for when `git2` has been phased out, i.e. everything else is working, I think (unscientifically) there might be benefits in using worktrees for checkouts. Admittedly I don't know the history of why they weren't used in the first place. Also: `gitoxide` doesn't yet support local clones and might not have to if worktrees were used instead.