Retain existing array in RepeatedField.Clear

Fixes #7828.

(Also tweaks the comment for Capacity.)

csharp/src/Google.Protobuf.Test/Collections/RepeatedFieldTest.cs

@@ -888,5 +888,17 @@ public void Capacity_Zero()
             Assert.DoesNotThrow(() => list.Capacity = 0, "Can set Capacity to 0");
             Assert.AreEqual(0, list.Capacity);
         }
+
+        [Test]
+        public void Clear_CapacityUnaffected()
+        {
+            var list = new RepeatedField<int> { 1 };
+            Assert.AreEqual(1, list.Count);
+            Assert.AreEqual(8, list.Capacity);
+
+            list.Clear();
+            Assert.AreEqual(0, list.Count);
+            Assert.AreEqual(8, list.Capacity);
+        }
     }
 }

csharp/src/Google.Protobuf/Collections/RepeatedField.cs

@@ -279,8 +279,9 @@ public void WriteTo(ref WriteContext ctx, FieldCodec<T> codec)
         }
 
         /// <summary>
-        /// Gets and sets the capacity of the RepeatedField's internal array.  WHen set, the internal array is reallocated to the given capacity.
-        /// <exception cref="ArgumentOutOfRangeException">The new value is less than Count -or- when Count is less than 0.</exception>
+        /// Gets and sets the capacity of the RepeatedField's internal array.
+        /// When set, the internal array is reallocated to the given capacity.
+        /// <exception cref="ArgumentOutOfRangeException">The new value is less than <see cref="Count"/>.</exception>
         /// </summary>
         public int Capacity
         {
@@ -338,7 +339,10 @@ public void Add(T item)
         /// </summary>
         public void Clear()
         {
-            array = EmptyArray;
+            // Clear the content of the array (so that any objects it referred to can be garbage collected)
+            // but keep the capacity the same. This allows large repeated fields to be reused without
+            // array reallocation.
+            Array.Clear(array, 0, count);
             count = 0;
         }