Wrap git/hg detectors in try-catch. See #905.

[cpojer]

In #905 it was reported that people without mercurial had some trouble with jest -o. I fixed that initially but on some system it seems like childProcess.spawn can throw. It doesn't seem to happen on OS X locally but based on what I read about childProcess.spawn I do believe it can throw. I think it is best to wrap it in a try-catch now – the worst thing that can happen is to report back as not a git or not an hg repo, which it won't be if the commands fail.

I also took a note to write an integration test for this, now that we have integration tests. I will add a few in a follow up.

Test Plan:
Run jest -o with changed files both in a git and hg repo.

[cpojer]

@facebook-github-bot import

[ghost]

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