RFR: vcs: handle clean renames in combined diff parser

[Erik Helin]

Hi all,

please review this patch that makes the combined diff parser handle "clean"
renames with regards to both parents. A clean rename is when the file is just
renamed, no lines in the files have changed. The problem is that git does not
output a `diff --combined` line, nor any hunks, for a clean rename. The combined
diff parser has been updated to detect this situation and then return empty hunk
lists.

I also added a bunch of state checks to the combined diff parser to help
diagnose future issues faster.

Testing:
- `make test` passes on Linux x64
- Manual testing with large merge commits

Thanks,
Erik

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Commit messages:
 - vcs: handle clean renames in combined diff parser

Changes: https://git.openjdk.java.net/skara/pull/549/files
 Webrev: https://webrevs.openjdk.java.net/skara/549/webrev.00
  Stats: 52 lines in 1 file changed: 45 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/549.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/549/head:pull/549

PR: https://git.openjdk.java.net/skara/pull/549