.gitattributes: set diff driver for Rust source code files

Git supports a builtin Rust diff driver [1] since v2.23.0 (2019).

It improves the choice of hunk headers in some cases, such as
diffs within methods, since those are indented in Rust within
an `impl` block, and therefore the default diff driver would
pick the outer `impl` block instead (rather than the method
where the changed code is).

For instance, with the default diff driver:

    @@ -455,6 +455,8 @@ impl fmt::Write for RawFormatter {
             // Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
             let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);

    +        test_diff_driver();
    +
             if len_to_copy > 0 {
                 // SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
                 // yet, so it is valid for write per the type invariants.

With the Rust diff driver:

    @@ -455,6 +455,8 @@ fn write_str(&mut sel...