Download commit pack even when commit exists as loose object

When TryDownloadCommit finds the commit via CommitAndRootTreeExists,
it now checks whether the commit is a loose object. Loose commits
(e.g., from a prior 'git show' or 'git log' in a mounted enlistment)
do not include reachable trees. Skipping the download in this case
causes 'git checkout -f' to fail with 'unable to read tree', followed
by an expensive fallback that re-downloads and retries checkout.

If the commit is in a pack file (prefetch or commit pack), trees are
included by the GVFS protocol, so the download can safely be skipped.

Added GitRepo.LooseObjectExists() to check whether a SHA exists as a
loose object file in the shared cache or local object store.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv

GVFS/GVFS.Common/Git/GitRepo.cs

@@ -101,6 +101,35 @@ public virtual bool CommitAndRootTreeExists(string commitSha, out string rootTre
             return output;
         }
 
+        /// <summary>
+        /// Check whether a given object SHA exists as a loose object file
+        /// in the shared cache or local object store.
+        /// </summary>
+        public virtual bool LooseObjectExists(string sha)
+        {
+            if (GVFSPlatform.Instance.Constants.CaseSensitiveFileSystem)
+            {
+                sha = sha.ToLower();
+            }
+
+            string looseObjectPath = Path.Combine(
+                this.enlistment.GitObjectsRoot,
+                sha.Substring(0, 2),
+                sha.Substring(2));
+
+            if (this.fileSystem.FileExists(looseObjectPath))
+            {
+                return true;
+            }
+
+            looseObjectPath = Path.Combine(
+                this.enlistment.LocalObjectsRoot,
+                sha.Substring(0, 2),
+                sha.Substring(2));
+
+            return this.fileSystem.FileExists(looseObjectPath);
+        }
+
         public virtual bool ObjectExists(string blobSha)
         {
             bool output = false;

GVFS/GVFS/CommandLine/GVFSVerb.cs

@@ -487,15 +487,33 @@ protected bool TryDownloadCommit(
             out string error,
             bool checkLocalObjectCache = true)
         {
-            if (!checkLocalObjectCache || !repo.CommitAndRootTreeExists(commitId, out _))
+            if (checkLocalObjectCache && repo.CommitAndRootTreeExists(commitId, out _))
             {
-                if (!gitObjects.TryDownloadCommit(commitId))
+                if (repo.LooseObjectExists(commitId))
                 {
-                    error = "Could not download commit " + commitId + " from: " + Uri.EscapeDataString(objectRequestor.CacheServer.ObjectsEndpointUrl);
-                    return false;
+                    // The commit exists as a loose object (e.g., from a prior 'git show'
+                    // or 'git log' in a mounted enlistment). Loose commits do not include
+                    // their reachable trees — those would need to be fetched individually.
+                    // Download the commit pack which includes all reachable trees so that
+                    // operations like 'git checkout -f' can succeed without the read-object
+                    // hook.
+                }
+                else
+                {
+                    // The commit exists in a pack file (prefetch pack or a previous commit
+                    // pack download). Packs from the GVFS protocol include all reachable
+                    // trees, so we can safely skip re-downloading.
+                    error = null;
+                    return true;
                 }
             }
 
+            if (!gitObjects.TryDownloadCommit(commitId))
+            {
+                error = "Could not download commit " + commitId + " from: " + Uri.EscapeDataString(objectRequestor.CacheServer.ObjectsEndpointUrl);
+                return false;
+            }
+
             error = null;
             return true;
         }