RFR: webrev: refactor webrev stats

[Erik Helin]

Hi all,

please review this patch that renames some of the stats methods (`added`, `modified`, `removed`) in `Hunk`, `Patch` and
`Diff`. I want to rename them to clarify that they are statistics tailored for webrev (which has a rather peculiar
notion of what a modified line is).

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - webrev: refactor webrev stats

Changes: https://git.openjdk.java.net/skara/pull/713/files
 Webrev: https://webrevs.openjdk.java.net/skara/713/webrev.00
  Stats: 422 lines in 17 files changed: 174 ins; 170 del; 78 mod
  Patch: https://git.openjdk.java.net/skara/pull/713.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/713/head:pull/713

PR: https://git.openjdk.java.net/skara/pull/713