fix(collect-updates): Avoid improper bumps from prompt selections

Fixes lerna#1357

Huge thanks to all the contributors on that issue, especially @Adam13531
for their exceptionally detailed reproduction and @randy-askin for their
clear (and patient!) identification of the ternary bug.

collect-updates.js

@@ -56,10 +56,11 @@ function collectUpdates(filteredPackages, packageGraph, execOpts, commandOptions
     candidates = new Set();
 
     const hasDiff = makeDiffPredicate(committish, execOpts, commandOptions.ignoreChanges);
-    const needsBump = (commandOptions.bump || "").startsWith("pre")
-      ? () => false
-      : /* skip packages that have not been previously prereleased */
-        node => node.prereleaseId;
+    const needsBump =
+      !commandOptions.bump || commandOptions.bump.startsWith("pre")
+        ? () => false
+        : /* skip packages that have not been previously prereleased */
+          node => node.prereleaseId;
 
     packages.forEach((node, name) => {
       if (forced.has(name) || needsBump(node) || hasDiff(node)) {