Add guard against changed files

If the file changes between when the lint was triggered and the start of
output processing simply tell Linter to not update the messages instead of
attempting to mark a document in a different state from when the output
was generated.

Author: Arcanemagus

lib/main.js

@@ -29,6 +29,7 @@ export default {
       lintOnFly: true,
       lint: async (textEditor) => {
         const filePath = textEditor.getPath();
+        const fileText = textEditor.getText();
         const fileExtension = extname(filePath).substr(1);
 
         if (this.ignoredExtensions.includes(fileExtension)) {
@@ -37,10 +38,14 @@ export default {
 
         const execArgs = ['-wc', '-E utf-8'];
         const execOpts = {
-          stdin: textEditor.getText(),
+          stdin: fileText,
           stream: 'stderr',
         };
         const output = await helpers.exec(this.executablePath, execArgs, execOpts);
+        if (textEditor.getText() !== fileText) {
+          // File contents have changed, just tell Linter not to update messages
+          return null;
+        }
         const toReturn = [];
         let match = regex.exec(output);
         while (match !== null) {