Added a new test checking that the incremental loads really only
fetch new commits.
On an incremental load of chromium.git:
before:
```
Enumerating objects: 18141323, done.
Counting objects: 100% (4755/4755), done.
Compressing objects: 100% (2733/2733), done.
ERROR:swh.loader.git.loader.GitLoader:Loading failure, updating to `failed` status
```
after:
```
Enumerating objects: 1203181, done.
Counting objects: 100% (407/407), done.
Compressing objects: 100% (233/233), done.
Total 1203181 (delta 270), reused 279 (delta 174), pack-reused 1202774
```