RFR: git combined diffs suffer from underflow

[Erik Duveblad via github.com]

Hi all,

seems like `git diff -c --unified=0` also suffers from underflow, but _not_ the off-by-one bug for empty hunks that `git diff --unified=0` suffers from. I updated the code to work around this issue.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64

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Commits:
 - 1a1813f7:	git combined diffs suffer from underflow

Pull request:
https://git.openjdk.java.net/skara/pull/53

Webrev:
https://webrevs.openjdk.java.net/skara/53/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/53.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/53/head:pull/53