PERF: improves performance in GroupBy.cumcount
closes #12839 

```
    -------------------------------------------------------------------------------
    Test name                                    | head[ms] | base[ms] |  ratio   |
    -------------------------------------------------------------------------------
    groupby_ngroups_10000_cumcount               |   3.9790 |  74.9559 |   0.0531 |
    groupby_ngroups_100_cumcount                 |   0.6043 |   0.9940 |   0.6079 |
    -------------------------------------------------------------------------------
    Test name                                    | head[ms] | base[ms] |  ratio   |
    -------------------------------------------------------------------------------

    Ratio < 1.0 means the target commit is faster then the baseline.
    Seed used: 1234

    Target [2a1c935] : PERF: improves performance in GroupBy.cumcount
    Base   [5b1f3b6] : reverts 'from .pandas_vb_common import *'
```
