If we change an upstream URL, all submodules break. Users would need
to run 'git submodule sync'. Note that the libbacktrace fix was merged
upstream so this is no longer necessary, but it's good for future changes.
Also, stress-testing reveals that git submodule fails locking
'.git/config' when run in paralell. It also segfaults and other
problems.
This is my final attempt to fix submodules; I've wasted far too many
days on obscure problems it creates: I've already lost one copy of my
repo to apparently unfixable submodule preoblems. The next "fix" will
be to simply import the source code so it works properly.
Reported-by: @jsarenik
Fixes: #1543
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
