RFR: Add --recurse-submodules=on-demand to git fetch commands

[Erik Helin]

Hi all,

please review this small patch that adds `--recurse-submodules=on-demand` to the `git fetch` commands in `GitRepository`. Adding `--recurse-submodules` means blobs for submodules will automatically fetched _if necessary_ (when a submodule has been updated in the parent repo). If no submodules are present then no additional action will be taken.

Thanks,
Erik

## Testing
- `make test` passes on Linux x64

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Commits:
 - ac45a86a: Add --recurse-submodules=on-demand to git fetch commands

Changes: https://git.openjdk.java.net/skara/pull/225/files
 Webrev: https://webrevs.openjdk.java.net/skara/225/webrev.00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/225.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/225/head:pull/225

PR: https://git.openjdk.java.net/skara/pull/225