RFR: Add config option sync.branches and make git-sync less chatty

[Erik Helin]

Hi all,

please review this small pull request that adds the configuration option `sync.branches`. Setting `sync.branches` means I don't have to provide `--branches` to `git-sync` every time for repositories where I know I always want to sync only a subset of the remote branches.

I also moved the printing of "Skipping branch foo..." behind the `--verbose` (or `--debug`) switch, since `git-sync` became quite chatty otherwise for upstream repositories with lots of branches.

Thanks,
Erik

## Testing
- [x] Manual testing of `git-sync`

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Commits:
 - a8c4dee6: Add config option sync.branches and make git-sync less chatty

Changes: https://git.openjdk.java.net/skara/pull/279/files
 Webrev: https://webrevs.openjdk.java.net/skara/279/webrev.00
  Stats: 11 lines in 1 file changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/279.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/279/head:pull/279

PR: https://git.openjdk.java.net/skara/pull/279