[Mount Removal] Remove shared cache mapping file

[wilbaker]

This is required to ensure that multiple clones of the same repo will use the same cache folder, even when the cache folders are deleted between the clones getting created:

1. Clone repo
2. Delete shared cache
3. Clone a second copy of the repo

Both copies should use the same shared cache location.

Rather than using a mapping file, the following approach can be used:

• Append vsts/info to the clone URL and check if there is a response.
• If there is a response: Use the repo ID in the response
• If there is not a response: Use a stable hash to compute a folder name from the URL

Additionally, drop a text file inside the repo specific cache directory that includes the name/URL of the repo. That file should get picked up by the diagnose verb.

[wilbaker]

Additionally, drop a text file inside the repo specific cache directory that includes the name/URL of the repo. That file should get picked up by the diagnose verb.

Splitting this work out into a separate work item as it is not a breaking disk layout change