Have copilot handle formatting better (#51114)

Author: marcpopMSFT

.github/copilot-instructions.md

@@ -5,6 +5,8 @@ Coding Style and Changes:
 - When generating code, run `dotnet format` to ensure uniform formatting.
 - Prefer using file-based namespaces for new code.
 - Do not allow unused `using` directives to be committed.
+- Commit your changes, and then format them.
+- Add the format commit SHA to the .git-blame-ignore-revs file so that the commit doesn't dirty git blame in the future
 - Use `#if NET` blocks for .NET Core specific code, and `#if NETFRAMEWORK` for .NET Framework specific code.
 
 Testing: