repo_manager: Allow unshallow to return non-zero. (#4963)
repo_manager: Allow unshallow to return non-zero. (#4963)

Unshallow can fail in a submodule, even if the unshallow succeeded for
the main repo.
skia-dev/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz