grep: turn off gitlink detection for --no-index

If we are running "git grep --no-index" outside of a git repository, we behave roughly like "grep -r", examining all files in the current directory and its subdirectories. However, because we use fill_directory() to do the recursion, it will skip over any directories which look like sub-repositories. For a normal git operation (like "git grep" in a repository) this makes sense; we do not want to cross the boundary out of our current repository into a submodule. But for "--no-index" without a repository, we should look at all files, including embedded repositories. There is one exception, though: we probably should _not_ descend into ".git" directories. Doing so is inefficient and unlikely to turn up useful hits. This patch drops our use of dir.c's gitlink-detection, but we do still avoid ".git". That makes us more like tools such as "ack" or "ag", which also know to avoid cruft in .git. As a bonus, this also drops our usage of the ref code when we are outside of a repository, making the transition to pluggable ref backends cleaner. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>