dev: removed zero-transfer rebase

contracts/mocks/ERC20Rebasing.vy

@@ -63,9 +63,12 @@ def allowance(_owner: address, _spender: address) -> uint256:
 
 @external
 def transfer(_to: address, _value: uint256) -> bool:
-    self._rebase()
     _shares: uint256 = self._get_shares_by_coins(_value)
 
+    if _shares > 0:
+        # only rebase on nonzero transfers
+        self._rebase()
+
     self.shares[msg.sender] -= _shares
     self.shares[_to] += _shares
     log Transfer(msg.sender, _to, _value)
@@ -74,9 +77,13 @@ def transfer(_to: address, _value: uint256) -> bool:
 
 @external
 def transferFrom(_from: address, _to: address, _value: uint256) -> bool:
-    self._rebase()
     _shares: uint256 = self._get_shares_by_coins(_value)
     _shares = min(self.shares[_from], _shares)
+
+    if _shares > 0:
+        # only rebase on nonzero transfers
+        self._rebase()
+
     # Value can be less than expected even if self.shares[_from] > _shares
     _new_value: uint256 = self._get_coins_by_shares(_shares)
 
@@ -99,20 +106,20 @@ def approve(_spender: address, _value: uint256) -> bool:
 @view
 def _share_price() -> uint256:
     if self.totalShares == 0:
-        return 10 ** self.decimals
-    return self.totalCoin * 10 ** self.decimals / self.totalShares
+        return 10**self.decimals
+    return self.totalCoin * 10**self.decimals / self.totalShares
 
 
 @internal
 @view
 def _get_coins_by_shares(_shares: uint256) -> uint256:
-    return _shares * self._share_price() / 10 ** self.decimals
+    return _shares * self._share_price() / 10**self.decimals
 
 
 @internal
 @view
 def _get_shares_by_coins(_coins: uint256) -> uint256:
-    return _coins * 10 ** self.decimals / self._share_price()
+    return _coins * 10**self.decimals / self._share_price()
 
 
 @external