RFR: skara: explicitly update submodules

[Erik Helin]

Hi all,

please review this patch that makes the `git skara update` explicitly update
eventual submodules for better compatibility with older git clients.

Thanks,
Erik

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Commit messages:
 - skara: explicitly update submodules

Changes: https://git.openjdk.java.net/skara/pull/550/files
 Webrev: https://webrevs.openjdk.java.net/skara/550/webrev.00
  Stats: 19 lines in 4 files changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/550.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/550/head:pull/550

PR: https://git.openjdk.java.net/skara/pull/550