RFR: 197: git-fork should use --dissociate with --reference-if-able

[Erik Helin]

Hi all,

please review this patch that ensures we use the `--dissociate` option to `git
clone` when someone is using `git fork --reference`. We need to use this flag
to ensure that the objects are only borrowed from the referenced repository to
reduce network transfer, we don't the borrow to continue after the clone.

Testing:
- [x] Manual testing of `git-pr fork --reference`

Thanks,
Erik

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

Commits:
 - 03ae9043: skara-197

Changes: https://git.openjdk.java.net/skara/pull/322/files
 Webrev: https://webrevs.openjdk.java.net/skara/322/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-197
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/322.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/322/head:pull/322

PR: https://git.openjdk.java.net/skara/pull/322