git_config(): not having a per-repo config file is not an error

git_config(): not having a per-repo config file is not an error

Currently git_config() returns an error if there is no repo config file
available (cwd is not a git repo); it will correctly parse the system
and global config files, but still return an error.

It doesn't affect anything else since almost nobody is checking for the
return code (with the exception of 'git remote update').

A reorganization in 'git config' would benefit from being able to
properly detect errors in git_config() without the noise generated when
cwd is not a git repo.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>