git svn: read global+system config for clone+init

git svn: read global+system config for clone+init

Since $GIT_DIR does not exist when initializing new repositories,
we can follow back to the global and system config files for
git.

The logic for this was originally introduced when
$GIT_DIR/config was the only config file git could read (back
when "git config" was "git repo-config"), so the function is
renamed to "read_git_config" instead of "read_repo_config".

Signed-off-by: Eric Wong <normalperson@yhbt.net>