RFR: 2415: Update GitCommits::close to make sure it terminates all processes when an exception is thrown

[Zhao Song]

As Pavel pointed out in the pull request of [SKARA-2410](https://bugs.openjdk.org/browse/SKARA-2410)(https://github.com/openjdk/skara/pull/1697), with current implementation , GitCommits::close may have created multiple processes and only destroy one of them.

As Erik and Pavel suggested, we should make sure close() is called on each process.

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Commit messages:
 - SKARA-2415

Changes: https://git.openjdk.org/skara/pull/1699/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1699&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2415
  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1699.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1699/head:pull/1699

PR: https://git.openjdk.org/skara/pull/1699