Fix last diff line not allowing suggestions

Fixes a bug where suggestion line would be -1

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

@@ -126,7 +126,7 @@ export default {
       const diffLines = this.diffFile[INLINE_DIFF_LINES_KEY];
       let isAdding = false;
 
-      for (let i = 0, diffLinesLength = diffLines.length - 1; i < diffLinesLength; i += 1) {
+      for (let i = 0, diffLinesLength = diffLines.length - 1; i <= diffLinesLength; i += 1) {
         const line = diffLines[i];
 
         if (start.line_code === line.line_code) {

app/assets/javascripts/vue_shared/components/markdown/field.vue

@@ -168,6 +168,9 @@ export default {
         false,
       );
     },
+    suggestionsStartIndex() {
+      return Math.max(this.lines.length - 1, 0);
+    },
   },
   watch: {
     isSubmitting(isSubmitting) {
@@ -260,7 +263,7 @@ export default {
       :line-content="lineContent"
       :can-suggest="canSuggest"
       :show-suggest-popover="showSuggestPopover"
-      :suggestion-start-index="lines.length - 1"
+      :suggestion-start-index="suggestionsStartIndex"
       @preview-markdown="showPreviewTab"
       @write-markdown="showWriteTab"
       @handleSuggestDismissed="() => $emit('handleSuggestDismissed')"

spec/frontend/snippets/components/__snapshots__/snippet_description_edit_spec.js.snap

@@ -29,7 +29,7 @@ exports[`Snippet Description Edit component rendering matches the snapshot 1`] =
     >
       <markdown-header-stub
         linecontent=""
-        suggestionstartindex="-1"
+        suggestionstartindex="0"
       />
        
       <div