Ensure --no-prefetch clone is honored by maintenance

The first prefetch for a large repository can take some time and (if gvfs.trust-
pack-indexes is false) heavy CPU and disk resources.

If a user cloned with --no-prefetch, let's assume they knew what they were doing
and not automatically prefetch after 15 minutes.

A typical use-case for --no-prefetch is in a CI pipeline which only needs the
head of a branch.

Author: tyrielv

GVFS/GVFS.Common/Maintenance/PrefetchStep.cs

@@ -16,6 +16,7 @@ public class PrefetchStep : GitMaintenanceStep
         private const int LockWaitTimeMs = 100;
         private const int WaitingOnLockLogThreshold = 50;
         private const string PrefetchCommitsAndTreesLock = "prefetch-commits-trees.lock";
+        private const int NoExistingPrefetchPacks = -1;
         private readonly TimeSpan timeBetweenPrefetches = TimeSpan.FromMinutes(70);
 
         public PrefetchStep(GVFSContext context, GitObjects gitObjects, bool requireCacheLock = true)
@@ -80,6 +81,18 @@ protected override void PerformMaintenance()
                 return;
             }
 
+            if (last == NoExistingPrefetchPacks)
+            {
+                /* If there are no existing prefetch packs, that means that either the
+                 * first prefetch is still in progress or the clone was run with "--no-prefetch".
+                 * In either case, we should not run prefetch as a maintenance task.
+                 * If users want to prefetch after cloning with "--no-prefetch", they can run
+                 * "gvfs prefetch" manually. Also, "git pull" and "git fetch" will run prefetch
+                 * as a pre-command hook. */
+                this.Context.Tracer.RelatedInfo(this.Area + ": Skipping prefetch since there are no existing prefetch packs");
+                return;
+            }
+
             DateTime lastDateTime = EpochConverter.FromUnixEpochSeconds(last);
             DateTime now = DateTime.UtcNow;
 
@@ -150,7 +163,7 @@ private bool TryGetMaxGoodPrefetchTimestamp(out long maxGoodTimestamp, out strin
                 .OrderBy(packInfo => packInfo.Timestamp)
                 .ToList();
 
-            maxGoodTimestamp = -1;
+            maxGoodTimestamp = NoExistingPrefetchPacks;
 
             int firstBadPack = -1;
             for (int i = 0; i < orderedPacks.Count; ++i)