Requires git 2.11, which we should have everywhere.
The new behavior is to caches the submodules in deploy-cache/cache/modules/, then
when cloning to revs/$rev/ we use --recurse-submodules and --reference ../cache/
then git does the magic to make the clone's submodules reuse the cached objects.
Disk usage, using rPHDEP as an example.
Cache modules
$ du -hs cache/.git/modules/ 121M cache/.git/modules/
Checkout in revs/
$ du -hs revs/test/..git/modules
2.6M revs/test/.git/modules