Generate source file without leading slash

Our code to write translations back to disk when we create pull requests
does not handle leading slahses well and anyway, the sourceFile is meant
to be relative, not absolute.

webapp/src/store/ProjectStore.spec.ts

@@ -140,6 +140,37 @@ describe('ProjectStore', () => {
     });
   });
 
+  describe('updateTranslation', () => {
+    it('does not prefix source file with /', async () => {
+      const store = new ProjectStore(
+        {
+          getMessages: async () => [
+            {
+              defaultMessage: 'Click',
+              id: 'core.click',
+              params: [],
+            },
+          ],
+        },
+        {
+          getTranslations: async () => ({
+            en: {
+              'core.click': {
+                sourceFile: 'en.yml',
+                text: 'Click',
+              },
+            },
+          }),
+        },
+      );
+
+      await store.updateTranslation('sv', 'core.click', 'Klicka');
+
+      const actual = await store.getTranslations('sv');
+      expect(actual['core.click'].sourceFile).toEqual('sv.yml');
+    });
+  });
+
   it('gives full access to all languages', async () => {
     const msgAdapter = mockMsgAdapter();
     msgAdapter.getMessages.mockResolvedValue([

webapp/src/store/ProjectStore.ts

@@ -100,6 +100,7 @@ export class ProjectStore {
       /^en\.(.+\.)*(ya?ml)$/g,
       `${lang}.$1$2`,
     );
-    return enSourceFileArr.join('/').concat(`/${langFileName}`);
+    enSourceFileArr.push(langFileName);
+    return enSourceFileArr.join('/');
   }
 }