std/hash_map: fix ensureUnusedCapacity() over-allocating

Currently this function adds the desired unused capactiy to the current
total capacity instead of the current used capactiy.

lib/std/hash_map.zig

@@ -848,7 +848,7 @@ pub fn HashMapUnmanaged(
             return ensureUnusedCapacityContext(self, allocator, additional_size, undefined);
         }
         pub fn ensureUnusedCapacityContext(self: *Self, allocator: *Allocator, additional_size: Size, ctx: Context) !void {
-            return ensureTotalCapacityContext(self, allocator, self.capacity() + additional_size, ctx);
+            return ensureTotalCapacityContext(self, allocator, self.count() + additional_size, ctx);
         }
 
         pub fn clearRetainingCapacity(self: *Self) void {
@@ -1956,6 +1956,19 @@ test "std.hash_map getOrPutAdapted" {
     }
 }
 
+test "std.hash_map ensureUnusedCapacity" {
+    var map = AutoHashMap(u64, u64).init(testing.allocator);
+    defer map.deinit();
+
+    try map.ensureUnusedCapacity(32);
+    const capacity = map.capacity();
+    try map.ensureUnusedCapacity(32);
+
+    // Repeated ensureUnusedCapacity() calls with no insertions between
+    // should not change the capacity.
+    try testing.expectEqual(capacity, map.capacity());
+}
+
 test "compile everything" {
     std.testing.refAllDecls(AutoHashMap(i32, i32));
     std.testing.refAllDecls(StringHashMap([]const u8));