setup: split off a function to handle ordinary .git directories

setup: split off a function to handle ordinary .git directories

Finish the clean-up of setup_git_directory_gently() by splitting the
last case of validation+setup (global variables, prefix, check_format,
set_git_dir) into its own function.  Now setup_git_git_directory_gently
itself takes care of discovery only and the functions that pick up
from there are nearby in the source file so they can be easily
compared.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>