CommitGraphWriter: fix UnusedException errorprone error

CommitGraphWriter: fix UnusedException errorprone error

Errorprone run in the bazel build raised this exception:

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java:105:
error: [UnusedException] This catch block catches an exception and
re-throws another, but swallows the caught exception rather than setting
it as a cause. This can make debugging harder.
		} catch (InterruptedIOException e) {
		  ^
    (see https://errorprone.info/bugpattern/UnusedException)
  Did you mean 'throw new
IOException(JGitText.get().commitGraphWritingCancelled, e);'?

Change-Id: Iad832fe17955fc1e60e6a4902bc50fd9dca76b9d