mergechanges: Fix #923427 (handling of multi-line Binary and missing packages in Binary)

mergechanges: Use Dpkg::Control to extract fields

This makes mergechanges support a multi-line Binary field.

Closes: #923427

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mergechanges: Tolerate packages in Files not listed in Binary

Packages built on stretch appear to have these for their -dbgsym packages.

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test_mergechanges: Test the sample changes file from #923427