RFR: git-fork: add --sync option

[Erik Helin]

Hi all,

please review this patch that adds the `--sync` option to `git-fork`. Setting
`--sync` (either via command-line or configuration file) makes `git-fork` sync
the fork with the upstream repository as a last step. This ensures that the
local clone of the fork on the user's machine is fully up to date with
upstream.

Note that I hade to shuffle some code around in `git-sync` to allow for this,
since `git-fork` simply just calls `git-sync` to implement this feaure.

Thanks,
Erik

- [x] Manual testing of `git-fork`

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

Commits:
 - e234fb97: git-fork: add --sync option

Changes: https://git.openjdk.java.net/skara/pull/304/files
 Webrev: https://webrevs.openjdk.java.net/skara/304/webrev.00
  Stats: 36 lines in 2 files changed: 27 ins; 4 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/304.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/304/head:pull/304

PR: https://git.openjdk.java.net/skara/pull/304