Bind EUNMERGED exception as IndexContainsUnmergedEntriesException

Here is another new exception to wrap the native `EUNMERGED` one.
Throwing this exception was not yet covered by tests, so I added some.
I copied the [mergedrepo](https://github.com/libgit2/libgit2/tree/development/tests-clar/resources/mergedrepo) from LibGit2 test resources to have a test repository with unmerged entries. this new `MergedTestRepo` works the same way as the `StandardTestRepo`.

So that I could add 2 tests :
* Soft reset a repository with unmerged entries throws 
`IndexContainsUnmergedEntriesException`
* Commit against a repo with unmerged entries throws `IndexContainsUnmergedEntriesException`