SourceLink with GitHub Enterprise LDAP appears to fail

I've enabled a given internal Github Enterprise project with sourcelink (specifically 1.0.0-beta-63127-02). The sourcelink itself appears to be generated successfully.

When I load this package into a new console app made in Visual Studio 2017 Preview 15.8 with the new option `Fall back to Git Credential Manager authentication for all Source Link requests` enabled so that it will [work with Github Enterprise](https://developercommunity.visualstudio.com/content/problem/257347/unnecessary-restrictions-on-authenticated-source-l.html?childToView=257351), I get the following error from VS:

```
Source Link Error:
git-credential-manager.exe: Using credentials for username "myuser".
ERROR: The request returned an unexpected mime type (text/html).

Source Link URL: https://git.mycompany.com/raw/MyOrg/MyRepo/3875a6f3507fb3c4e5a727d49fe09328dba97a14/src/MyProject/MyFile.cs
```

It appears that the URL generated by sourcelink just returns a `200` response with a login page & a redirect.


Approximate URL is `https://git.mycompany.com/login?return_to=https%3A%2F%2Fgit.mycompany.com%2Fraw%2FMyOrg%2FMyRepo%2F3875a6f3507fb3c4e5a727d49fe09328dba97a14%2Fsrc%2FMyProject%2FMyFile.cs`

It's unclear if this is a bug in SourceLink, Visual Studio or Github Enterprise. Advice appreciated on debugging this further or what we should correct to make this exciting integration work.