Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Bump debhelper from old 10 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Repository-Browse. (upstream-metadata-file-is-missing)

These changes have no impact on the binary debdiff.

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Build and test logs for this branch can be found at https://janitor.debian.net/run/b64a35a4-21d9-4eff-8cd2-b95e6e6e0de3.