Stop reduce subcommand if it’s not making progress

If the chunk sizes are really tiny compared to the reduced source file, it looks like we aren't making any progress reducing. Just abort the reduction in that case.

Author: ahoppen

Sources/swift-parser-test/swift-parser-test.swift

@@ -265,6 +265,10 @@ class Reduce: ParsableCommand {
     var reduced = source
     var chunkSize = source.count / 4
     while chunkSize > 0 {
+      if chunkSize < reduced.count / 20 {
+        // The chunk sizes are really tiny compared to the source file. Looks like we aren't making any progress reducing. Abort.
+        break
+      }
       if verbose {
         printerr("Current source size \(reduced.count), reducing with chunk size \(chunkSize)")
       }