BaseTools/PatchCheck.py: Fix error when run with Python3

BaseTools/PatchCheck.py: Fix error when run with Python3

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1145

Currently run PatchCheck.py with Python3 will meet
following error:
.....
File "PatchCheck.py", line 554, in run_git
return Result[0].decode('utf-8', 'ignore') if Result[0] and
Result[0].find("fatal")!=0 else None
TypeError: a bytes-like object is required, not 'str'

This issue was introduce by commit:5ac4548cdf654.

This patch is to convert the str object of "fatal" to
byte object to fix this failure.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>