Parallelize pre-mount validations with network operations

Start auth + config query immediately on entry to Mount(), before repo
metadata loading. This overlaps network latency with all local I/O:
metadata, git version/hooks/filesystem checks, and git config writes.

The network task (auth + config) and local task (validations + git config)
run concurrently via Task.WhenAll. RepoMetadata loading runs on the main
thread between task launch and local task start, overlapping with the
initial anonymous auth probe.

Measured improvement on os.2020 (2.4M index entries):
  Production:  ~29s wall clock
  Sequential:  ~22s (prior commit moved work to mount.exe)
  Parallel:    ~19s (this commit)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tyrielv

GVFS/GVFS.Mount/InProcessMount.cs

@@ -18,6 +18,7 @@
 using System.Security;
 using System.Text;
 using System.Threading;
+using System.Threading.Tasks;
 using static GVFS.Common.Git.LibGit2Repo;
 
 namespace GVFS.Mount
@@ -84,6 +85,27 @@ public void Mount(EventLevel verbosity, Keywords keywords)
         {
             this.currentState = MountState.Mounting;
 
+            // Start auth + config query immediately — these are network-bound and don't
+            // depend on repo metadata or cache paths. Every millisecond of network latency
+            // we can overlap with local I/O is a win.
+            Stopwatch parallelTimer = Stopwatch.StartNew();
+
+            var networkTask = Task.Run(() =>
+            {
+                Stopwatch sw = Stopwatch.StartNew();
+                string authError;
+                if (!this.enlistment.Authentication.TryInitialize(this.tracer, this.enlistment, out authError))
+                {
+                    this.tracer.RelatedWarning("Mount will proceed, but new files cannot be accessed until GVFS can authenticate: " + authError);
+                }
+
+                this.tracer.RelatedInfo("ParallelMount: Auth completed in {0}ms", sw.ElapsedMilliseconds);
+
+                ServerGVFSConfig config = this.QueryAndValidateGVFSConfig();
+                this.tracer.RelatedInfo("ParallelMount: Auth + config query completed in {0}ms", sw.ElapsedMilliseconds);
+                return config;
+            });
+
             // We must initialize repo metadata before starting the pipe server so it
             // can immediately handle status requests
             string error;
@@ -122,39 +144,57 @@ public void Mount(EventLevel verbosity, Keywords keywords)
 
             this.enlistment.InitializeCachePaths(localCacheRoot, gitObjectsRoot, blobSizesRoot);
 
-            if (!this.enlistment.Authentication.TryInitialize(this.tracer, this.enlistment, out error))
+            // Local validations and git config run while we wait for the network
+            var localTask = Task.Run(() =>
             {
-                this.tracer.RelatedWarning("Mount will proceed, but new files cannot be accessed until GVFS can authenticate: " + error);
-            }
+                Stopwatch sw = Stopwatch.StartNew();
 
-            this.ValidateGitVersion();
-            this.ValidateHooksVersion();
-            this.ValidateFileSystemSupportsRequiredFeatures();
+                this.ValidateGitVersion();
+                this.tracer.RelatedInfo("ParallelMount: ValidateGitVersion completed in {0}ms", sw.ElapsedMilliseconds);
 
-            ServerGVFSConfig serverGVFSConfig = this.QueryAndValidateGVFSConfig();
+                this.ValidateHooksVersion();
+                this.ValidateFileSystemSupportsRequiredFeatures();
 
-            CacheServerResolver cacheServerResolver = new CacheServerResolver(this.tracer, this.enlistment);
-            this.cacheServer = cacheServerResolver.ResolveNameFromRemote(this.cacheServer.Url, serverGVFSConfig);
+                GitProcess git = new GitProcess(this.enlistment);
+                if (!git.IsValidRepo())
+                {
+                    this.FailMountAndExit("The .git folder is missing or has invalid contents");
+                }
 
-            this.EnsureLocalCacheIsHealthy(serverGVFSConfig);
+                if (!GVFSPlatform.Instance.FileSystem.IsFileSystemSupported(this.enlistment.EnlistmentRoot, out string fsError))
+                {
+                    this.FailMountAndExit("FileSystem unsupported: " + fsError);
+                }
 
-            GitProcess git = new GitProcess(this.enlistment);
-            if (!git.IsValidRepo())
-            {
-                this.FailMountAndExit("The .git folder is missing or has invalid contents");
-            }
+                this.tracer.RelatedInfo("ParallelMount: Local validations completed in {0}ms", sw.ElapsedMilliseconds);
+
+                if (!this.TrySetRequiredGitConfigSettings())
+                {
+                    this.FailMountAndExit("Unable to configure git repo");
+                }
 
-            if (!GVFSPlatform.Instance.FileSystem.IsFileSystemSupported(this.enlistment.EnlistmentRoot, out error))
+                this.LogEnlistmentInfoAndSetConfigValues();
+                this.tracer.RelatedInfo("ParallelMount: Local validations + git config completed in {0}ms", sw.ElapsedMilliseconds);
+            });
+
+            try
             {
-                this.FailMountAndExit("FileSystem unsupported: " + error);
+                Task.WaitAll(networkTask, localTask);
             }
-
-            if (!this.TrySetRequiredGitConfigSettings())
+            catch (AggregateException ae)
             {
-                this.FailMountAndExit("Unable to configure git repo");
+                this.FailMountAndExit(ae.Flatten().InnerExceptions[0].Message);
             }
 
-            this.LogEnlistmentInfoAndSetConfigValues();
+            parallelTimer.Stop();
+            this.tracer.RelatedInfo("ParallelMount: All parallel tasks completed in {0}ms", parallelTimer.ElapsedMilliseconds);
+
+            ServerGVFSConfig serverGVFSConfig = networkTask.Result;
+
+            CacheServerResolver cacheServerResolver = new CacheServerResolver(this.tracer, this.enlistment);
+            this.cacheServer = cacheServerResolver.ResolveNameFromRemote(this.cacheServer.Url, serverGVFSConfig);
+
+            this.EnsureLocalCacheIsHealthy(serverGVFSConfig);
 
             using (NamedPipeServer pipeServer = this.StartNamedPipe())
             {