use the branch_name as the default for the optional commit hash

There was an issue with the script when passing optional rustfmt configs
without specifying a commit hash. Because these optional values are
passed via positional arguments the configs ($4)  would be used in place
of the commit hash ($3). Now that we set a default value for the
optional commit hash we avoid this problem.

.github/workflows/check_diff.yml

@@ -30,4 +30,4 @@ jobs:
         rustup target add x86_64-unknown-linux-gnu
 
     - name: check diff
-      run: bash ${GITHUB_WORKSPACE}/ci/check_diff.sh ${{ github.event.inputs.clone_url }} ${{ github.event.inputs.branch_name }} ${{ github.event.inputs.commit_hash }} ${{ github.event.inputs.rustfmt_configs }}
+      run: bash ${GITHUB_WORKSPACE}/ci/check_diff.sh ${{ github.event.inputs.clone_url }} ${{ github.event.inputs.branch_name }} ${{ github.event.inputs.commit_hash || github.event.inputs.branch_name }} ${{ github.event.inputs.rustfmt_configs }}

ci/check_diff.sh

@@ -115,7 +115,7 @@ function compile_rustfmt() {
     git fetch feature $FEATURE_BRANCH
 
     cargo build --release --bin rustfmt && cp target/release/rustfmt $1/rustfmt
-    if [ -z "$OPTIONAL_COMMIT_HASH" ]; then
+    if [ -z "$OPTIONAL_COMMIT_HASH" ] || [ "$FEATURE_BRANCH" = "$OPTIONAL_COMMIT_HASH" ]; then
         git switch $FEATURE_BRANCH
     else
         git switch $OPTIONAL_COMMIT_HASH --detach