fix: only run cargo fmt hook when rs files changed (#2620)

### Description

The `cargo fmt` pre-commit hook runs regardless of whether any `.rs`
files have changed and this is interfering with unrelated workflows.
This reduces the scope of the hook to commits with nonempty rust diffs.

### Drive-by changes

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### Related issues

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- Fixes #[issue number here]
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### Backward compatibility

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implications, e.g. changes that would prohibit deploying older commits
using this infra tooling?

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### Testing

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.husky/pre-commit

@@ -2,4 +2,9 @@
 . "$(dirname -- "$0")/_/husky.sh"
 
 yarn lint-staged
-cargo fmt --all --check --manifest-path rust/Cargo.toml
+
+# if any *.rs files have changed
+if git diff --staged --exit-code --name-only | grep -q -E ".*\.rs$"; then
+    echo "Running cargo fmt pre-commit hook"
+    cargo fmt --all --check --manifest-path rust/Cargo.toml
+fi