Add 'git submodule sync --recursive' to PlugUpdate

Some plugins use several nested git submodules (e.g. YouCompleteMe). If the URL of one of these submodules changes, PlugUpdate fails. Also `git submodule update --init --recursive` fails in the command line in this case:

> error: Server does not allow request for unadvertised object ...
> Fetched in submodule path '...', but it did not contain .... Direct fetching of that commit failed.
> Failed to recurse into submodule path '...'

The fix is to call `git submodule sync --recursive` prior to `git submodule update --init --recursive` in PlugUpdate.

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- Type:
- [x] Bug
- [ ] Enhancement
- [ ] Feature Request
- [ ] Question
- OS:
- [x] All/Other
- [ ] Linux
- [ ] OS X
- [ ] Windows
- Vim:
- [x] Terminal Vim
- [x] GVim
- [x] Neovim