Fix GitHub Enterprise API URL for raw source code links

[ejball]

When storing credentials for raw URLs of GitHub Enterprise instances (as Visual Studio does when Fall back to Git Credential Manager authentication for all Source Link requests is enabled), the GitHub Enterprise API hostname does not necessarily match the raw URL hostname. For example, a raw URL of https://raw.my.ghe.dev/... should access the API at https://my.ghe.dev/api/v3. For our purposes, removing raw. from the hostname would fix the issue, but perhaps some sort of configuration is necessary in the general case.

[ejball]

Source Link supports this via the ContentUrl attribute of SourceLinkGitHubHost (GHE usage mentioned here).

[mjcheetham]

This should have been fixed after #599 was merged. This feature will go out in the next GCM release - watch this space!