DfsFsck: Check that .gitmodules in the repository have valid contents

DfsFsck: Check that .gitmodules in the repository have valid contents

Previous commits block the addition to the repo of dangerous .gitmodules
files, but some could have been committed before those safeguards where
in place.

Add a check in DfsFsck to validate the .gitmodules files in the repo.
Use the same validation than the ReceivePack, translating the
results to FsckErrors.

Note that *all* .gitmodules files in the storage will be checked, not
only the latest version.

Change-Id: I040cf1f31a779419aad0292ba5e6e76eb7f32b66
Signed-off-by: Ivan Frade <ifrade@google.com>