RFR: fork: fix helptext for --reference

[Erik Helin]

Hi all,

please review this patch updates the helptext for the flag `--reference` of `git-fork` to say that `git-fork` will
always use `--reference-if-able` together with `--dissociate` when cloning (for safety reasons).

Thanks,
Erik

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Commit messages:
 - fork: fix helptext for --reference

Changes: https://git.openjdk.java.net/skara/pull/774/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=774&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/774.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/774/head:pull/774

PR: https://git.openjdk.java.net/skara/pull/774