ci: check formatting of our Rust code

Introduce a CI check that verifies that our Rust code is well-formatted.
This check uses `cargo fmt`, which is a wrapper around rustfmt(1) that
executes formatting for all Rust source files. rustfmt(1) itself is the
de-facto standard for formatting code in the Rust ecosystem.

The rustfmt(1) tool allows to tweak the final format in theory. In
practice though, the Rust ecosystem has aligned on style "editions".
These editions only exist to ensure that any potential changes to the
style don't cause reformats to existing code bases. Other than that,
most Rust projects out there accept this default style of a specific
edition.

Let's do the same and use that default style. It may not be anyone's
favorite, but it is consistent and by making it part of our CI we also
enforce it right from the start.

Note that we don't have to pick a specific style edition here, as the
edition is automatically derived from the edition we have specified in
our "Cargo.toml" file.

The implemented script looks somewhat weird as we perfom manual error
handling instead of using something like `set -e`. The intent here is
that subsequent commits will add more checks, and we want to execute all
of these checks regardless of whether or not a previous check failed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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