nix-prefetch-git: fix extraction of submodule hashes on latest git

Summary:

According to git-submodule manpage,
"git submodule status" prefixes the hash with a '-' if it is not
initialized, and other chars in other circumstances.
(this is consistent on the various git versions tested)

nix-prefetch-git runs "git submodule init" which does you'd think,
but apparently despite this earlier versions of git before 2.16
would still give the hash the '-' suffix.
In particular this is the behavior when using 2.15 and 2.14.1
from the nixos-17.09 and nixos-17.03 channels respectively.

The script then used awk to drop the first char of the first field
which does the wrong thing when there is no prefix emitted:
while there is a space character before the hash, this is not
part of the field and so we ended up eating the first character
of the hash.

To fix this in a way that also works with the previous behavior,
this commit instead uses awk to grab the hash field
and uses tr to delete any '-' chars should they be present.

This seems to work in my testing, and for example can now
successfully fetch the source for "nginxModules.brotli"
where previously it would generate an error:

fatal: '22564a95d9ab58865a096b8d9f7324ea5f2e03e' is not a commit and a branch 'fetchgit' cannot be created from it

(we dropped a '2' from the beginning of the hash)