RFR: 204: git-sync: must fetch before --fast-forward

[Erik Helin]

Hi all,

please review this patch that ensures `git sync --fast-forward` fetches the
changes from the `to` remote _before_ trying to do "fast forward" merges. This
is needed because doing a push to a remote ref does not cause that ref to be
updated.

Thanks,
Erik

## Testing
- [x] Manual testing of `git sync --fast-foward`
- [x] Added a new unit test

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

Commits:
 - 7694fb6e: skara-204

Changes: https://git.openjdk.java.net/skara/pull/331/files
 Webrev: https://webrevs.openjdk.java.net/skara/331/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-204
  Stats: 43 lines in 5 files changed: 41 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/331.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/331/head:pull/331

PR: https://git.openjdk.java.net/skara/pull/331