RFR: 2410: Introduce process killing finally block in GitCommits

[Zhao Song]

Recently, I found that for pr-external container, in GitCommits, the exit of the git process could time out and then the process will be stuck in sleeping state and eat memory. With Pavel's suggestion, I think we should introduce the process killing finally block.

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Commit messages:
 - SKARA-2410

Changes: https://git.openjdk.org/skara/pull/1697/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1697&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2410
  Stats: 32 lines in 1 file changed: 22 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/skara/pull/1697.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1697/head:pull/1697

PR: https://git.openjdk.org/skara/pull/1697