Fix multiline context showing file start and end - #709

--multi-line-context-lines is meant to keep context around each change in a multiline value. For leading and trailing equal hunks it also printed the beginning and end of the file.

Only keep the context adjacent to actual inserts/deletes. Equal hunks between changes still show both ends with the unchanged summary.

Example with --multi-line-context-lines 2:

Before:

  image:
    registry: custom-registry

  [two lines unchanged)]

  autoscaling:
    enabled: true
-   minReplicas: 2
+   minReplicas: 3
    maxReplicas: 10
    metrics: {}

  [two lines unchanged)]

      cpu: 100m
  priorityClassName: my-priority

After:

  autoscaling:
    enabled: true
-   minReplicas: 2
+   minReplicas: 3
    maxReplicas: 10
    metrics: {}

Fixes #686

Test plan

• go test ./...
• Regression for --multi-line-context-lines adds unneeded beginning and end of file #686 in pkg/dyff (assets under assets/issues/issue-686/)
• Manual: dyff between from.yml to.yml --output github --multi-line-context-lines 2 --omit-header