Checked out PR file diffs sometimes complain about different encodings

[jcansdale]

• GitHub Extension for Visual Studio version: 2.2.1.100 (dev)
  Visual Studio version: 2015

1. Select a PR
2. Checkout PR
3. Double-click on PR file and open diff view

Sometimes the diff view complains that the files have different encodings (UTF-8 vs Windows).

[jcansdale]

It appears Visual Studio likes to add UTF-8 headers to files it touches. 😕 This seems to change them from being the default Western European (Windows-1252) to being UTF-8.

When creating a temp file to diff against, we should save it as UTF-8, if the working file has been changed to UTF-8.

[jcansdale]

Fixed in #1011