The pre-submit test fails intermittently

During solving the issue SKARA-1254, I found the pre-submit test failed[1]. So I try to re-run it. The pre-submit test passed at this time[2].

The corresponding failed log is shown below:

```
> Task :jcheck:test FAILED
120 tests completed, 3 failed

JCheckTests > checkRemoval(VCS) > org.openjdk.skara.jcheck.JCheckTests.checkRemoval(VCS)[2] FAILED
    java.io.UncheckedIOException: java.io.IOException: Num headers (1) differ from num hunks (0)
        at org.openjdk.skara.vcs/org.openjdk.skara.vcs.hg.HgCommitIterator.next(HgCommitIterator.java:83)
        at org.openjdk.skara.vcs/org.openjdk.skara.vcs.hg.HgCommitIterator.next(HgCommitIterator.java:34)
        at org.openjdk.skara.jcheck/org.openjdk.skara.jcheck.iterators.MapIterator.next(MapIterator.java:45)
        at org.openjdk.skara.jcheck/org.openjdk.skara.jcheck.iterators.FlatMapIterator.hasNext(FlatMapIterator.java:46)
        at org.openjdk.skara.jcheck/org.openjdk.skara.jcheck.iterators.ConcatIterator.hasNext(ConcatIterator.java:39)
        at org.openjdk.skara.jcheck/org.openjdk.skara.jcheck.JCheckTests.checkRemoval(JCheckTests.java:295)

        Caused by:
        java.io.IOException: Num headers (1) differ from num hunks (0)
            at org.openjdk.skara.vcs/org.openjdk.skara.vcs.tools.GitRawDiffParser.parse(GitRawDiffParser.java:212)
            at org.openjdk.skara.vcs/org.openjdk.skara.vcs.hg.HgCommitIterator.next(HgCommitIterator.java:71)
            ... 5 more
Test report disabled, omitting generation of the HTML test report.
:jcheck:test (Thread[Daemon worker,5,main]) completed. Took 22.882 secs.
Gradle Test Executor 14 finished executing tests.
```

It seems the bug is only in mac and may be related to concurrency.

[1] https://github.com/lgxbslgx/skara/actions/runs/1525457193/attempts/1
[2] https://github.com/lgxbslgx/skara/actions/runs/1525457193/attempts/2