shortlog: run setup_git_directory_gently() sooner

shortlog: run setup_git_directory_gently() sooner

shortlog already runs a repository search unconditionally;
running such a search earlier is not very risky.

Without this change, the “[pager] shortlog” configuration
is not respected at all: “git shortlog” unconditionally paginates.

The tests are a bit slow.  Running the full battery like this
for all built-in commands would be counterproductive; the intent is
rather to test shortlog as a representative example command using
..._gently().

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>