git: pass in repo to builtin based on setup_git_directory_gently

git: pass in repo to builtin based on setup_git_directory_gently

The current code in run_builtin() passes in a repository to the builtin
based on whether cmd_struct's option flag has RUN_SETUP.

This is incorrect, however, since some builtins that only have
RUN_SETUP_GENTLY can potentially take a repository.
setup_git_directory_gently() tells us whether or not a command is being
run inside of a repository.

Use the output of setup_git_directory_gently() to help determine whether
or not there is a repository to pass to the builtin. If not, then we
just pass NULL.

As part of this patch, we need to modify add to check for a NULL repo
before calling repo_git_config(), since add -h can be run outside of a
repository.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>