Fix memory leak from not releasing the memory mapped file pointer

Author: AArnott

src/NerdBank.GitVersioning/ManagedGit/GitPackIndexMappedReader.cs

@@ -134,6 +134,7 @@ public override (long? Offset, GitObjectId? ObjectId) GetOffset(Span<byte> objec
     /// <inheritdoc/>
     public override void Dispose()
     {
+        this.accessor?.SafeMemoryMappedViewHandle.ReleasePointer();
         this.accessor?.Dispose();
         this.accessor = null;
         this.file.Dispose();
@@ -146,6 +147,7 @@ private unsafe ReadOnlySpan<byte> GetSpan(ulong offset, int length)
             // If the request is for a window that we have not currently mapped, throw away what we have.
             if (this.accessor is not null && (this.accessorOffset > offset || this.accessorOffset + this.accessorSize < offset + (ulong)length))
             {
+                this.accessor.SafeMemoryMappedViewHandle.ReleasePointer();
                 this.accessor.Dispose();
                 this.accessor = null;
             }