git wrapper: allow setup_git_directory_gently() be called earlier

git wrapper: allow setup_git_directory_gently() be called earlier

In the spirit of v1.4.2-rc3~34^2^2 (Call setup_git_directory() much
earlier, 2006-07-28), let run_builtin() take care of searching for a
repository for built-ins that want to make use of one if present.

So now you can mark your command with RUN_SETUP_GENTLY and use

	nongit = !startup_info->have_repository;

in place of

	prefix = setup_git_directory_gently(&nongit);

and everything will be the same, except the repository is
discovered a little sooner.

As v1.7.2~16^2 (2010-07-14) explains, this should allow more commands
to robustly use features like "git --paginate" that look at local
configuration before the command is actually run.

This patch sets up the infrastructure.  Later patches will teach
particular commands to use it.

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>