Support log.showSignature = true in .gitconfig

[anitahammer]

If user has a setting to show GPG signatures, Copybara breaks:

[log]
    showSignature = true

java.lang.IllegalArgumentException: Chunk [ture made Mon 06 May 2024 03:15:22 PM UTC] is not a valid entry
  at com.google.common.base.Preconditions.checkArgument(Preconditions.java:218)
  at com.google.common.base.Splitter$MapSplitter.split(Splitter.java:526)
  at com.google.copybara.git.GitRepository$LogCmd.parseLog(GitRepository.java:2428)
  at com.google.copybara.git.GitRepository$LogCmd.run(GitRepository.java:2404)
  at com.google.copybara.git.ChangeReader.run(ChangeReader.java:108)
  at com.google.copybara.git.GitOrigin$ReaderImpl.changes(GitOrigin.java:506)
  at com.google.copybara.git.GitOrigin$ReaderImpl.changes(GitOrigin.java:291)
  at com.google.copybara.WorkflowRunHelper.getChanges(WorkflowRunHelper.java:285)
  at com.google.copybara.WorkflowMode$1.run(WorkflowMode.java:84)
  at com.google.copybara.Workflow.run(Workflow.java:296)
  at com.google.copybara.MigrateCmd.run(MigrateCmd.java:92)
  at com.google.copybara.MigrateCmd.run(MigrateCmd.java:69)
  at com.google.copybara.Main.runInternal(Main.java:246)
  at com.google.copybara.Main.run(Main.java:128)
  at com.google.copybara.Main.main(Main.java:106)
ERROR: Unexpected error (please file a bug against copybara): Chunk [ture made Mon 06 May 2024 03:15:22 PM UTC] is not a valid entry (java.lang.IllegalArgumentException: Chunk [ture made Mon 06 May 2024 03:15:22 PM UTC] is not a valid entry)

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