Fix 2+ discussions on one line not working

Always create a lines object, even if only one line is provided.

Some instances of creating the note form won't have a range of lines
at construction.

If only a single line is provided, create the range of lines using
that one line as both the start and end of the range.

Changelog: fixed

app/assets/javascripts/diffs/components/diff_line_note_form.vue

@@ -149,7 +149,11 @@ export default {
     },
   },
   created() {
-    this.lines = { ...this.range };
+    if (this.range) {
+      this.lines = { ...this.range };
+    } else if (this.line) {
+      this.lines = { start: this.line, end: this.line };
+    }
   },
   mounted() {
     if (this.isLoggedIn) {

spec/frontend/diffs/components/diff_line_note_form_spec.js

@@ -24,11 +24,14 @@ describe('DiffLineNoteForm', () => {
     return shallowMount(DiffLineNoteForm, {
       store,
       propsData: {
-        diffFileHash: diffFile.file_hash,
-        diffLines,
-        line: diffLines[1],
-        range: { start: diffLines[0], end: diffLines[1] },
-        noteTargetLine: diffLines[1],
+        ...{
+          diffFileHash: diffFile.file_hash,
+          diffLines,
+          line: diffLines[1],
+          range: { start: diffLines[0], end: diffLines[1] },
+          noteTargetLine: diffLines[1],
+        },
+        ...(args.props || {}),
       },
     });
   };
@@ -119,6 +122,22 @@ describe('DiffLineNoteForm', () => {
     });
   });
 
+  describe('created', () => {
+    it('should use the provided `range` of lines', () => {
+      wrapper = createComponent();
+
+      expect(wrapper.vm.lines.start).toBe(diffLines[0]);
+      expect(wrapper.vm.lines.end).toBe(diffLines[1]);
+    });
+
+    it("should fill the internal `lines` data with the provided `line` if there's no provided `range", () => {
+      wrapper = createComponent({ props: { range: null } });
+
+      expect(wrapper.vm.lines.start).toBe(diffLines[1]);
+      expect(wrapper.vm.lines.end).toBe(diffLines[1]);
+    });
+  });
+
   describe('mounted', () => {
     it('should init autosave', () => {
       const key = 'autosave/Note/Issue/98//DiffNote//1c497fbb3a46b78edf04cc2a2fa33f67e3ffbe2a_1_2';