added 'explicit' keyword to Bitmap constructor

sql/ha_isammrg.cc:
  no Bitmap::operator=(ulonglong) present

sql/ha_isammrg.cc

@@ -157,7 +157,7 @@ void ha_isammrg::info(uint flag)
   deleted = (ha_rows) info.deleted;
   data_file_length=info.data_file_length;
   errkey  = info.errkey;
-  table->keys_in_use=0;				// No keys yet
+  table->keys_in_use.clear_all();               // No keys yet
   table->db_options_in_use    = info.options;
   mean_rec_length=info.reclength;
   block_size=0;

sql/sql_bitmap.h

@@ -29,7 +29,7 @@ template <uint default_width> class Bitmap
 public:
   Bitmap() { init(); }
   Bitmap(Bitmap& from) { *this=from; }
-  Bitmap(uint prefix_to_set) { init(prefix_to_set); }
+  explicit Bitmap(uint prefix_to_set) { init(prefix_to_set); }
   void init() { bitmap_init(&map, buffer, default_width, 0); }
   void init(uint prefix_to_set) { init(); set_prefix(prefix_to_set); }
   uint length() const { return default_width; }
@@ -91,7 +91,7 @@ template <> class Bitmap<64>
   ulonglong map;
 public:
   Bitmap<64>() { }
-  Bitmap<64>(uint prefix_to_set) { set_prefix(prefix_to_set); }
+  explicit Bitmap<64>(uint prefix_to_set) { set_prefix(prefix_to_set); }
   void init() { }
   void init(uint prefix_to_set) { set_prefix(prefix_to_set); }
   uint length() const { return 64; }