ceph-pr-submodules: do not use shallow-clone

ceph-pr-submodules: do not use shallow-clone

we use
```
git diff --submodule=log origin/$ghprbTargetBranch...$ghprbActualCommit
```

for checking the differences between the delta since their common
ancestor, so we have to do a non-shallow clone for accessing the
common ancestor.

Signed-off-by: Kefu Chai <kchai@redhat.com>