BUG/TST: TimeGrouper has erroneous groups if key length is too short
BUG/TST: TimeGrouper has erroneous groups if key length is too short

closes #8542

Author: sinhrks <sinhrks@gmail.com>

Closes #13028 from sinhrks/tgrouper_groups and squashes the following commits:

93b41d5 [sinhrks] BUG: TimeGrouper has erroneous groups if key length is too short

DGrady/pandas,pratapvardhan/pandas,nmartensen/pandas,cbertinato/pandas,pratapvardhan/pandas,zfrenchee/pandas,amolkahat/pandas,pandas-dev/pandas,cbertinato/pandas,MJuddBooth/pandas,Winand/pandas,harisbal/pandas,winklerand/pandas,gfyoung/pandas,linebp/pandas,pratapvardhan/pandas,dsm054/pandas,louispotok/pandas,pandas-dev/pandas,TomAugspurger/pandas,GuessWhoSamFoo/pandas,dsm054/pandas,gfyoung/pandas,dsm054/pandas,winklerand/pandas,winklerand/pandas,harisbal/pandas,jmmease/pandas,jmmease/pandas,DGrady/pandas,gfyoung/pandas,datapythonista/pandas,dsm054/pandas,pratapvardhan/pandas,jorisvandenbossche/pandas,louispotok/pandas,nmartensen/pandas,harisbal/pandas,TomAugspurger/pandas,pandas-dev/pandas,pratapvardhan/pandas,louispotok/pandas,datapythonista/pandas,linebp/pandas,jmmease/pandas,MJuddBooth/pandas,jmmease/pandas,linebp/pandas,nmartensen/pandas,DGrady/pandas,cython-testbed/pandas,kdebrab/pandas,rs2/pandas,jreback/pandas,jorisvandenbossche/pandas,pandas-dev/pandas,winklerand/pandas,louispotok/pandas,toobaz/pandas,gfyoung/pandas,cbertinato/pandas,cbertinato/pandas,datapythonista/pandas,Winand/pandas,DGrady/pandas,Winand/pandas,kdebrab/pandas,louispotok/pandas,toobaz/pandas,jreback/pandas,linebp/pandas,nmartensen/pandas,kdebrab/pandas,nmartensen/pandas,jmmease/pandas,zfrenchee/pandas,kdebrab/pandas,GuessWhoSamFoo/pandas,GuessWhoSamFoo/pandas,winklerand/pandas,jorisvandenbossche/pandas,winklerand/pandas,linebp/pandas,toobaz/pandas,TomAugspurger/pandas,harisbal/pandas,amolkahat/pandas,datapythonista/pandas,cython-testbed/pandas,Winand/pandas,jreback/pandas,rs2/pandas,toobaz/pandas,cbertinato/pandas,amolkahat/pandas,harisbal/pandas,toobaz/pandas,Winand/pandas,GuessWhoSamFoo/pandas,amolkahat/pandas,DGrady/pandas,jmmease/pandas,rs2/pandas,jreback/pandas,Winand/pandas,jreback/pandas,MJuddBooth/pandas,jorisvandenbossche/pandas,cython-testbed/pandas,linebp/pandas,zfrenchee/pandas,nmartensen/pandas,kdebrab/pandas,cython-testbed/pandas,cython-testbed/pandas,TomAugspurger/pandas,DGrady/pandas,amolkahat/pandas,MJuddBooth/pandas,gfyoung/pandas,dsm054/pandas,zfrenchee/pandas,MJuddBooth/pandas,GuessWhoSamFoo/pandas,rs2/pandas,zfrenchee/pandas