Always sync submodules before updating them

This ensures the origin in `.git/config` matches the one in
`.gitmodules`.  Git will quite appropriately refrain from doing this
automatically, because it never allows remote repositories to update
local config.  You have to ask.

(See: https://stackoverflow.com/a/45679261)

In Vundle's case, it is always correct to sync.  These aren't repos that
a developer maintains; they are effectively read-only copies of remote
state.  Since syncing is always correct, and git won't sync unless we
ask, then we should always sync.

Fixes VundleVim#911.

autoload/vundle/installer.vim

@@ -394,6 +394,7 @@ func! s:make_sync_command(bang, bundle) abort
                   \ 'git remote set-url origin ' . vundle#installer#shellesc(a:bundle.uri),
                   \ 'git fetch',
                   \ 'git reset --hard origin/HEAD',
+                  \ 'git submodule sync --recursive',
                   \ 'git submodule update --init --recursive',
                   \ ]
       let cmd = join(cmd_parts, ' && ')
@@ -410,6 +411,7 @@ func! s:make_sync_command(bang, bundle) abort
     let cmd_parts = [
                 \ 'cd '.vundle#installer#shellesc(a:bundle.path()),
                 \ 'git pull',
+                \ 'git submodule sync --recursive',
                 \ 'git submodule update --init --recursive',
                 \ ]
     let cmd = join(cmd_parts, ' && ')