I don't understand two things in the output of git diff:
$ git diff 8bdc f5fa
diff --git a/text.md b/text.md
index 4202174..6ef3a44 100644
@@ -3,3 +3,4 @@ title: Foo
+Once upon a time...
1. What does the line with the word "index" mean?
2. What does the line with the @@ mean?
How are these @@ called? Do they hava a name?
What does the numbers between the @@ mean? They refer somehow to the
changed lines, I guess. But what exactly do they say?
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