RFR: 1879: Make GitRepository#isEmptyCommit be compatible with old versions of Git

[Zhao Song]

In SKARA-1854, we introduced a method GitRepository#isEmptyCommit. In addition, the git show command includes the option '--diff-merges=dense-combined' in git 2.30.0. However, this option is not available in old versions of git. To ensure compatibility with old versions of Git, we should replace '--diff-merges=dense-combined' with the '--cc' option.

References:
[1]https://git-scm.com/docs/git-show/2.29.0
[2]https://git-scm.com/docs/git-show/2.30.0

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Commit messages:
 - SKARA-1879

Changes: https://git.openjdk.org/skara/pull/1501/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1501&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1879
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1501.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1501/head:pull/1501

PR: https://git.openjdk.org/skara/pull/1501