RFR: GitCommitIterator should handle empty commits

[Erik Duveblad via github.com]

HI all,

the `GitCommitIterator` should be able to handle empty commits. Although empty commits are rare, there are a few in the OpenJDK repositories.

## Testing
- [x] Added two new unit tests
- [x] `sh gradlew test` passes on Linux x86_64

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Commits:
 - e770a955:	GitCommitIterator should handle empty commits

Pull request:
https://git.openjdk.java.net/skara/pull/54

Webrev:
https://webrevs.openjdk.java.net/skara/54/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/54.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/54/head:pull/54