Finding the remote in submodules of Cargo’s checkouts of git dependencies

[SimonSapin]

Similar to #3, but this time for submodules:

No remotes found for Git repo /home/simon/.cargo/git/checkouts/fontsan-75864180be477bc9/29e879c/src/ots, packaging files instead...

git remote -v in that directory is empty, but this time the information is available in .gitmodules at the root of the parent repository. Still, it may be easier or nicer to modify Cargo’s use of libgit2 than to add hacks to pernosco-submit to find and parse .gitmodules.