Ignore checking formatting if no formatted files are staged (#1469)

I'm constantly harassed to follow the rules when I'm not doing anything
relevant.

This is an attempt to ameliorate that rather than `--no-verify` or just
disabling hooks altogether. 🍄 ☁️

Somebody might want to confirm `basename` is included with macOS.

.githooks/check-format

@@ -5,6 +5,20 @@
 # This script checks the format of the code and cmake files.
 # In many cases it will automatically fix the issues and abort the commit.
 
+no_formatted_directories_staged() {
+    staged_directories=$(git diff-index --cached --name-only HEAD | awk -F/ '{print $1}')
+    for sd in $staged_directories; do
+        if [[ "$sd" =~ ^(benchmark|cmake|src|tests)$ ]]; then
+            return 1
+        fi
+    done
+    return 0
+}
+
+if no_formatted_directories_staged ; then
+    exit 0
+fi
+
 echo "+ Checking code format..."
 
 # paths to check and re-format