RFR: 769: Replies to e-mail comments on GitHub are threaded incorrectly

[Robin Westberg]

If a comment is done in reply to a bridged email, setup reply-to headers correctly pointing to the source of the bridged comment.

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/1111/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1111&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-769
  Stats: 251 lines in 7 files changed: 210 ins; 29 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/1111.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1111/head:pull/1111

PR: https://git.openjdk.java.net/skara/pull/1111