Add ruby diff context to gitattribute

What does this MR do?

Add ruby diff scoping to .gitattribute, so diffing is more context aware. I learned this from https://tekin.co.uk/2020/10/better-git-diff-output-for-ruby-python-elixir-and-more

Before change:

@@ -26,6 +26,7 @@ class FooWorker
+      hihi

After change:

@@ -26,6 +26,7 @@ def perform
+      hihi

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides