Add a failing test for git_object_short_id

Hey guys!

The patch adds a currently failing test for `git_object_short_id`.

```
$ ./libgit2_clar -sobject::shortid
Loaded 241 suites:
Started
F

  1) Failure:
object::shortid::select [(...)/libgit2/tests/object/shortid.c:31]
  Function call failed: git_object_short_id(&shorty, obj)
  error -3 - Object not found - no matching pack entry for prefix (038d718000000000000000000000000000000000)
```

Git seems to have no problems with the object:

```
$ pwd
(...)/libgit2/tests/resources/duplicate.git
$ git cat-file -e 038d718da6a1ebbc6a7780a96ed75a70cc2ad6e2 && git cat-file -e 038d718
$ echo $?
0
$ git cat-file -p 038d718
testing
```

It seems like the error is coming from internals not necessarily related to `git_object_short_id` but this seemed like the easiest way to reproduce it.