Use faster setText when replacing entire buffer contents

Summary:`setText` is 3-4x faster than `setTextViaDiff`. Using it when replacing
the entire contents of the buffer makes clicking through files in the
diff-view tree show the file up to 100ms faster.

**Test run #1**

setText, 39.4ms:
{F60653266}

setTextViaDiff, 170.5ms:
{F60653267}

**Test run #2**

setText, 68.4ms:
{F60653269}

setTextViaDiff, 171.8ms:
{F60653270}

Reviewed By: mostafaeweda

Differential Revision: D3203887

fb-gh-sync-id: 7d78e07373d688654f6f6f723f30013ffe9e06fd
fbshipit-source-id: 7d78e07373d688654f6f6f723f30013ffe9e06fd

pkg/nuclide-diff-view/lib/DiffViewEditor.js

@@ -68,10 +68,19 @@ export default class DiffViewEditor {
   }
 
   setFileContents(filePath: string, contents: string): void {
-    // The text is set via diffs to keep the cursor position.
     const buffer = this._editor.getBuffer();
     if (buffer.getText() !== contents) {
-      buffer.setTextViaDiff(contents);
+      if (buffer.getPath() === filePath && !buffer.isEmpty()) {
+        // The text is set via diffs to keep the cursor position when updating the file that is
+        // already active.
+        buffer.setTextViaDiff(contents);
+      } else {
+        // `setText` is faster than diffing and is used for speed when the buffer is changing to a
+        // different path or when an editable buffer is being loaded for the first time
+        // (`buffer.isEmpty() === true`) because maintaining cursor position between file changes is
+        // not needed.
+        buffer.setText(contents);
+      }
     }
     const grammar = atom.grammars.selectGrammar(filePath, contents);
     this._editor.setGrammar(grammar);