Re: [PATCH 17/18] sha1_file: always allow relative paths to alternates

> We recursively expand alternates repositories, so that if A
> borrows from B which borrows from C, A can see all objects.
>
> For the root object database, we allow relative paths, so A
> can point to B as "../B/objects". However, we currently do
> not allow relative paths when recursing, so B must use an
> absolute path to reach C.
>
> That is an ancient protection from c2f493a (Transitively
> read alternatives, 2006-05-07) that tries to avoid adding
> the same alternate through two different paths. Since
> 5bdf0a8 (sha1_file: normalize alt_odb path before comparing
> and storing, 2011-09-07), we use a normalized absolute path
> for each alt_odb entry.
>
> This means that in most cases the protection is no longer
> necessary; we will detect the duplicate no matter how we got
> there (but see below).  And it's a good idea to get rid of
> it, as it creates an unnecessary complication when setting
> up recursive alternates (B has to know that A is going to
> borrow from it and make sure to use an absolute path).
>

> Note that our normalization doesn't actually look at the
> filesystem, so it can still be fooled by crossing symbolic
> links. But that's also true of absolute paths, so it's not a
> good reason to disallow only relative paths (it's
> potentially a reason to switch to real_path(), but that's a
> separate and non-trivial change).