concat ignore_index option doesnt work
#### Code Sample, a copy-pastable example if possible

```python
pd.concat([df_general.iloc[:,2:],df],ignore_index=True).fillna(method='ffill')
tried vs.
pd.concat([df_general.iloc[:,2:],df],ignore_index=False).fillna(method='ffill')



```
#### Problem description
concatenating the tables leaves 1 partially empty row that shouldnt be there, on pandas version version (0.23.4) it didn't happen.
this happened after upgrading pandas to below version.

#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.0.final.0
python-bits      : 64
OS               : Windows
OS-release       : 7
machine          : AMD64
processor        : Intel64 Family 6 Model 30 Stepping 4, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.16.3
pytz             : 2018.5
dateutil         : 2.7.3
pip              : 20.0.2
setuptools       : 41.0.1
Cython           : 0.28.5
pytest           : 3.8.0
hypothesis       : None
sphinx           : 1.7.9
blosc            : None
feather          : None
xlsxwriter       : 1.1.0
lxml.etree       : 4.2.5
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 6.5.0
pandas_datareader: None
bs4              : 4.6.3
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.5
matplotlib       : 2.2.3
numexpr          : 2.6.8
odfpy            : None
openpyxl         : 2.5.6
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 3.8.0
pyxlsb           : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.2.11
tables           : 3.4.4
tabulate         : None
xarray           : None
xlrd             : 1.1.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.0
numba            : 0.39.0
</details>
