RFR: skara update: fix bad Files.exists check

[Erik Helin]

Hi all,

please review this small patch that fixes an bad `Files.exists` check in `git
skara update`.

Testing:
- Manual testing of `git skara update`

Thanks,
Erik

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Commit messages:
 - skara update: fix bad Files.exists check

Changes: https://git.openjdk.java.net/skara/pull/562/files
 Webrev: https://webrevs.openjdk.java.net/skara/562/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/562.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/562/head:pull/562

PR: https://git.openjdk.java.net/skara/pull/562