fix: look also for previous prerelease versions to determine the next one

lib/get-next-version.js

@@ -14,7 +14,9 @@ module.exports = ({branch, nextRelease: {type, channel}, lastRelease, logger}) =
       ) {
         version = highest(
           semver.inc(lastRelease.version, 'prerelease'),
-          `${semver.inc(getLatestVersion(tagsToVersions(branch.tags)), type)}-${branch.prerelease}.${FIRSTPRERELEASE}`
+          `${semver.inc(getLatestVersion(tagsToVersions(branch.tags), {withPrerelease: true}), type)}-${
+            branch.prerelease
+          }.${FIRSTPRERELEASE}`
         );
       } else {
         version = `${semver.inc(`${major}.${minor}.${patch}`, type)}-${branch.prerelease}.${FIRSTPRERELEASE}`;

test/get-next-version.test.js

@@ -258,3 +258,20 @@ test('Increase version for release on prerelease branch based on highest commit
     '2.0.0-beta.1'
   );
 });
+
+test('Increase version for release on prerelease branch when there is no regular releases on other branches', t => {
+  t.is(
+    getNextVersion({
+      branch: {
+        name: 'beta',
+        type: 'prerelease',
+        prerelease: 'beta',
+        tags: [{gitTag: 'v1.0.0-beta.1', version: '1.0.0-beta.1', channels: ['beta']}],
+      },
+      nextRelease: {type: 'minor', channel: 'beta'},
+      lastRelease: {version: 'v1.0.0-beta.1', channels: ['beta']},
+      logger: t.context.logger,
+    }),
+    '1.0.0-beta.2'
+  );
+});