Failure wile `git submodule update --recursive --remote`

[cc32d9]

secp256k1/upstream points to a Github repo that does not have a master branch. This leads to an error when trying to update all externals recursively from my project that uses FC as external.

It also reproduces in standalone FC:

eosio@sthist:~/tmp$ git clone https://github.com/EOSIO/fc.git
Cloning into 'fc'...
remote: Enumerating objects: 155, done.
remote: Counting objects: 100% (155/155), done.
remote: Compressing objects: 100% (78/78), done.
remote: Total 2188 (delta 73), reused 115 (delta 57), pack-reused 2033
Receiving objects: 100% (2188/2188), 1.01 MiB | 2.68 MiB/s, done.
Resolving deltas: 100% (1174/1174), done.
eosio@sthist:~/tmp$ cd fc/
eosio@sthist:~/tmp/fc$ git submodule update --init --recursive
Submodule 'secp256k1/upstream' (https://github.com/cryptonomex/secp256k1-zkp.git) registered for path 'secp256k1/upstream'
Cloning into '/home/eosio/tmp/fc/secp256k1/upstream'...
Submodule path 'secp256k1/upstream': checked out 'bd067945ead3b514fba884abd0de95fc4b5db9ae'
eosio@sthist:~/tmp/fc$ git submodule update --init --recursive
eosio@sthist:~/tmp/fc$ git submodule update --recursive --remote
fatal: Needed a single revision
Unable to find current origin/master revision in submodule path 'secp256k1/upstream'
eosio@sthist:~/tmp/fc$