RFR: git-sync: add --fast-forward option

[Erik Helin]

Hi all,

please review this patch that adds the `--fast-forward` option to `git-sync`.
Setting the option `--fast-forward` will cause `git-sync` to fast-forward all
local branches where possible. Since it is mostly common that developer's do
not make change to branches in their personal forks that are also present in
the upstream repository, the effect is that all local branches that map to
upstream branches (via the personal fork) will be updated locally.

Thanks,
Erik

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

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Commits:
 - b82d9401: git-sync: add --fast-forward option

Changes: https://git.openjdk.java.net/skara/pull/309/files
 Webrev: https://webrevs.openjdk.java.net/skara/309/webrev.00
  Stats: 66 lines in 1 file changed: 61 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/309.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/309/head:pull/309

PR: https://git.openjdk.java.net/skara/pull/309