RFR: git-sync: use refspec instead of --prune-tags

[Erik Helin]

Hi all,

please review this small patch that uses a refspec instead of `--prune-tags` for
`GitRepository.fetchRemote`. The flag `--prune-tags` was introduced in Git 2.17
and we want to support older Git versions, so I opted to use a refspec instead
to achieve the same effect.

Testing:
- Manual testing of `git sync` on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - git-sync: do not use --prune-tags

Changes: https://git.openjdk.java.net/skara/pull/529/files
 Webrev: https://webrevs.openjdk.java.net/skara/529/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/529.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/529/head:pull/529

PR: https://git.openjdk.java.net/skara/pull/529