Add repository/commit attribute to nuspec format

Nuspecs currently include repository type and url, this should be extended to include the commit hash.

Example:
```xml
    <repository type="git"
               url="https://github.com/NuGet/NuGet.Client"
               commit="17caacc71d2cfa40f6ce10d4f0bbec8ea68f4cda" />
```

Pack should be updated to allow passing this in from a property in the same way the repo url is set.