Update expected result for a foreach test

libgit2 is giving the same result as git (1.7.0.4), but the expected target in the test differs, resulting in:
```
  1) Failure:
odb::foreach::foreach [/home/ubuntu/code/c/libgit2/tests-clar/odb/foreach.c:45]
  47 + 1640 != nobj
  1687 != 1689
```

I don't know anything about this test or this aspect of git, so someone please confirm!

I've (blindly) updated the expected result in the test to match what git says for this PR, so this test passes, just in case that's all that's needed. Cheers!