RFR: vcs: proper source paths for merge commits

[Erik Helin]

Hi all,

please review this patch that fixes an issue with parsing Git merge commits. Without the `--combined-all-paths` the
"RAW" diff format of `git diff --combined` will only print the resulting (target) path for a rename. This mean that
`patch.source().path()` would `Optional.empty()` for such a patch. This patch fixes this by passing the
`--combined-all-paths` flag to `git log` and updating the `GitCombinedDiffParser`.

Testing:
- [x] Added a new unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - vcs: proper source paths for merge commits

Changes: https://git.openjdk.java.net/skara/pull/675/files
 Webrev: https://webrevs.openjdk.java.net/skara/675/webrev.00
  Stats: 60 lines in 3 files changed: 54 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/675.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/675/head:pull/675

PR: https://git.openjdk.java.net/skara/pull/675