Handles Source Build patches during initialization/synchronization of the VMR

Expects a new property in source-mappings.json pointing to where the patches are located in the VMR (later they can be moved):

{
    "patchesPath": "src/installer/src/SourceBuild/tarball/patches",
    ...
}

Consider we're updating repo A from sha1 to sha2, the new synchronization algorithm is as follows:

1. Checkout VMR
2. Clone individual repo A to temp
3. For each file that has a patch in the VMR, copy its sha1 version from the clone to the VMR (to working tree only)
4. Prepare a diff from A for sha1..sha2 and apply it onto the VMR
5. Apply any Source Build patches associated with the repo
6. Commit all above to the VMR

Resolves dotnet/arcade#10261