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 #911.