only read start-devmode.sh once

This file was being read twice: once to hash it, and again for its
contents. The new function hashString() is now used to compute the hash
from the data already read.

services/service.ts

@@ -99,6 +99,15 @@ async function hashFile(filePath: string, algorithm: string): Promise<string> {
   return hash.read();
 }
 
+/**
+ * Hashes a string with specified algorithm.
+ *
+ * Input should be UTF-8.
+ */
+function hashString(data: string, algorithm: string): string {
+  return createHash(algorithm).update(data, 'utf-8').digest('hex');
+}
+
 /**
  * Elevates a package by name.
  */
@@ -525,11 +534,12 @@ function runService() {
 
         // RootMyTV v1
         if (await isFile(startDevmode)) {
-          const localChecksum = await hashFile(startDevmode, 'sha256');
           // Warn and return empty string on read error
           let startDevmodeContents = await asyncReadFile(startDevmode, { encoding: 'utf-8'} )
             .catch((err) => (console.warn(`reading ${startDevmode} failed: ${err.toString()}`), '')) as string;
 
+          const localChecksum = hashString(startDevmodeContents, 'sha256');
+
           if (localChecksum !== bundledStartupChecksum && updateableChecksums.indexOf(localChecksum) !== -1) {
             await copyScript(bundledStartup, startDevmode);
             messages.push(`${startDevmode} updated!`);