Step-by-step

1. Go to http://gnuwin32.sourceforge.net/packages/tree.htm
2. Download the Binary version (see image below)
3. Save the zip file to your computer
4. Open the zip file and double click to bin folder
5. Extract tree.exe to C:\Program Files\Git\usr\bin
6. Try on your git-bash now. Result

Credit:
https://superuser.com/questions/531592/how-do-i-add-the-tree-command-to-git-bash-on-windows/1141489#1141489