Preserve trailing commas

[webdevred]

Summary: Hpack removes trailing commas from multi-line lists, e.g., build-depends and extra-source-files. Related #617

Example:

Before:

  vector >=0.13,

After hpack:

  vector >=0.13

Expected behavior: Keep trailing commas when they exist in the original file.

Reason: Trailing commas reduce diff noise when adding/removing elements in a list.