Add gix-diff WASM check, even though gix-path covers it (#75)

As noted in #2092, the `wasm` jobs on CI do not test `gix-diff`
directly. However, my prediction there that breakage would not be
detected on CI was mistaken, because those jobs do test the `wasm`
feature of `gix-pack`. The `gix-pack` crate depends on `gix-diff`,
and its `wasm` feature enable the `gix-diff` one.

This nonetheless adds an explicit check for `gix-diff`. This
`gix-diff` check does not attempt to build default features, since
some fail on some WASM targets. But the preexisting `gix-pack`
check does still build the default features of `gix-pack`, which
are compatible with WASM targets.

The efficacy of these checks, as well as the need to pass
`--no-default-features` for `gix-diff`, can be confirmed by
examining CI results for various fragments of this change
experimented on in EliahKagan#75. This also demonstrates that CI is
capable of catching at least some breakages related to the `wasm`
feature of `gix-diff`, and thus may be sufficient to support moving
forward with #2092.

Author: EliahKagan

.github/workflows/ci.yml

@@ -460,9 +460,8 @@ jobs:
       - name: crates with 'wasm' feature
         run: |
           set -x
-          for crate in gix-pack; do
-            cargo build -p "$crate" --features wasm --target "$TARGET"
-          done
+          cargo build -p gix-diff --no-default-features --features wasm --target "$TARGET"
+          cargo build -p gix-pack --features wasm --target "$TARGET"
       - name: gix-pack with all features (including wasm)
         run: cargo build -p gix-pack --all-features --target "$TARGET"