RFR: vcs: fix hunk line parsing in UnifiedDiffParser

[Erik Helin]

Hi all,

please review this patch that fixes the parsing in `UnifiedDiffParser.parseSingleFileDiff` a bit. The parser now understands Git's extended headers and also properly moves past a hunk start lines (lines starting with `@@`).

Testing:
- [x] Added a new unit test

Thanks,
Erik

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Commit messages:
 - vcs: fix hunk line parsing in UnifiedDiffParser

Changes: https://git.openjdk.java.net/skara/pull/897/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=897&range=00
  Stats: 425 lines in 2 files changed: 423 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/897.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/897/head:pull/897

PR: https://git.openjdk.java.net/skara/pull/897