Provide ::git-ls-files:: to resolve git submodules

[idbrii]

Resolve #22: Cloned subdirectory fails

ls-tree doesn't support --recurse-submodules, but --ls-files does. So provide it as an option.

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This is a draft because I'm not sure how it handles symlinks. Also, I think before merging this pr, there should be tests for handling different cases. I don't understand why ls-tree is used instead of ls-files except maybe because it wasn't clear how to get only filenames (--abbrev=0 was definitely unobvious).