"TryDownloadPrefetchPacks (Start)" message is missing from prefetch logs

In the latest version of VFS4G, the prefetch log includes TryDownloadPrefetchPacks (Stop) but is missing TryDownloadPrefetchPacks (Start).

This may be a recent break (when I checked logs from older versions TryDownloadPrefetchPacks (Start) was present).

Example:

C:\scratch\testClone>gvfs clone https://gvfs.visualstudio.com/ci/_git/ForTests
Clone parameters:
  Repo URL:     https://gvfs.visualstudio.com/ci/_git/ForTests
  Branch:       Default
  Cache Server: Default
  Local Cache:  C:\.gvfsCache
  Destination:  C:\scratch\testClone\ForTests
Authenticating...Succeeded
Querying remote for config...Succeeded
Using cache server: None (https://gvfs.visualstudio.com/ci/_git/ForTests)
Cloning...Succeeded
Fetching commits and trees from origin (no cache server)...Succeeded
Validating repo...Succeeded
Mounting...Succeeded
Registering for automount...Succeeded

C:\scratch\testClone>type ForTests\.gvfs\logs\gvfs_prefetch_20191014_132313.log
[2019-10-14 13:23:13 -07:00] Prefetch (Start) {"Version":"1.0.19224.2","EnlistmentRoot":"C:\\scratch\\testClone\\ForTests","Remote":"https://gvfs.visualstudio.com/ci/_git/ForTests","ObjectsEndpoint":"https://gvfs.visualstudio.com/ci/_git/ForTests"}
[2019-10-14 13:23:13 -07:00] PerformPrefetch {"Commits":true,"Files":null,"Folders":null,"FileListFile":null,"FoldersListFile":null,"FilesFromStdIn":false,"FoldersFromStdIn":false,"HydrateFiles":false}
[2019-10-14 13:23:14 -07:00] RetryConfig_LoadedRetryConfig {"Area":"RetryConfig","Timeout":"00:00:30","MaxRetries":6,"Message":"RetryConfigLoaded"}
[2019-10-14 13:23:14 -07:00] NetworkResponse {"RequestId":4,"availableConnections":12,"CacheName":"","StatusCode":200,"ContentType":"text/html","connectionWaitTimeMS":"0.0010","responseWaitTimeMS":"62.3842"}
[2019-10-14 13:23:14 -07:00] DeserializePrefetchPacks (Start)
[2019-10-14 13:23:14 -07:00] DeserializePrefetchPacks (Stop) {"DurationMs":2}
[2019-10-14 13:23:14 -07:00] TryDownloadPrefetchPacks (Stop) {"Area":"GitObjects","Success":true,"Attempts":1,"BytesDownloaded":0,"DurationMs":70}
[2019-10-14 13:23:14 -07:00] Prefetch (Stop) {"DurationMs":1625}

C:\scratch\testClone>gvfs --version
GVFS 1.0.19224.2

I have confirmed the behavior is the same with a cache server configured as well.

The issue names the TryDownloadPrefetchPacks prefetch operation and the gvfs_prefetch log; start by tracing that entry point and comparing current logging with the supplied example. Restore the missing Start event and verify that a prefetch produces both TryDownloadPrefetchPacks (Start) and (Stop) entries.