Remove need for Polonius borrow checker in git_odb::compound::Db::locate(…)

[Byron]

Currently there is branching code in git_odb::compound::Db::locate(…) to workaround a borrowcheck shortcoming using the upcoming Polonius borrow checker.

gitoxide/git-odb/src/compound/locate.rs

Lines 32 to 42 in b317200

	// See 8c5bd095539042d7db0e611460803cdbf172beb0 for a commit that adds polonius and makes the proper version compile.
	// See https://stackoverflow.com/questions/63906425/nll-limitation-how-to-work-around-cannot-borrow-buf-as-mutable-more-than?noredirect=1#comment113007288_63906425
	// More see below! Of course we don't want to do the lookup twice… but have to until this is fixed or we compile nightly.
	#[cfg(not(feature = "polonius"))]
	if alternate.locate(id, buffer)?.is_some() {
	return alternate.locate(id, buffer);
	}
	#[cfg(feature = "polonius")]
	if let Some(object) = alternate.locate(id, buffer)? {
	return Ok(Some(object));
	}

Without the workaround, the lookup costs are doubled.

Using Polonius solves the problem but comes at a cost.

• It requires the nightly compiler (forcing gitoxide to be compiled with nightly if that code-path is chosen).
• The borrow checker performance is reduced and can 'explode' for certain crates like tinyvec.
• It's unclear when Polonius will be stable.

Here is @joshtriplett thoughts on how to resolve this:

When first initializing the Db and looking up all the alternates, rather than recursing, collect all of the alternates as a top-level Vec of (Packs, Loose), so that alternates themselves never have alternates. That way, you don't have to recurse, and there's only one level of alternates. Then, in the loop over alternates, rather than recursively calling locate, just inline the same code that looks in packs and loose, but without the check for alternates.
I think the simplest way to do that would be to have a version of at (at_no_alternates) that ignores alternates entirely, and then have the top-level at collect a deduplicated list of alternates and call at_no_alternates on each one.

Additional Information

Also have a look at how this is currently done for looking up objects in packs. The gist is to get an index of where the object is found first, bypassing the need for borrowing an output buffer, and fill the buffer if an index was found.

gitoxide/git-odb/src/compound/locate.rs

Lines 28 to 30 in b317200

	if let Some(object) = self.loose.locate(id)? {
	return Ok(Some(compound::Object::Loose(object)));
	}

[Byron]

• linked repo type with init
• linked repo locate()
• support for caches in locate()
• object-access experiment uses new repo
• docs