debian mapper: better process Vcs-Git field

Some `.dsc` files have a `Vcs-Git` field, next to `Vcs-Browser`. From `zsh_5.2-3.dsc`:

```
Vcs-Browser: https://anonscm.debian.org/cgit/collab-maint/zsh.git
Vcs-Git: git://anonscm.debian.org/collab-maint/zsh.git -b debian
```

Although these are somewhat processed by and stored as `vcs-url` it might be worth breaking it down a bit more (the type of vcs, and so on). Also, as the repositories mentioned in `Vcs-Git` field are where the Debian specific metadata lives (such as the `.dsc` itself) it might be worth treating it differently (and potentially use it to mine even more information).