This allows to use `gitoxide` for all fetch operations, boosting performance
for fetching the `crates.io` index by a factor of 2.2x, while being consistent
on all platforms.
For trying it, nightly builds of `cargo` can specify `-Zgitoxide=fetch`.
It's also possible to set the `__CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2=1` environment
variable (value matters), which is when `-Zgitoxide=none` can be used
to use `git2` instead.
Limitations
-----------
Note that what follows are current shortcomings that will be addressed in future PRs.
- it's likely that authentication around the `ssh` protocol will work differently in practice
as it uses the `ssh` program.
- clones from `file://` based crates indices will need the `git` binary to serve the locatl repository.
- the progress bar shown when fetching doesn't work like the orgiinal, but should already feel 'faster'.