builtin-add.c: optimize -A option and "git add ."

The earlier "git add -A" change was done in a quite inefficient way (i.e. it is as unefficient as "git add -u && git add ." modulo one fork/exec and read/write index). When the user asks "git add .", we do not have to examine all paths we encounter and perform the excluded() and dir_add_name() processing, both of which are slower code and use slower data structure by git standards, especially when the index is already populated. Instead, we implement "git add $pathspec..." as: - read the index; - read_directory() to process untracked, unignored files the current way, that is, recursively doing readdir(), filtering them by pathspec and excluded(), queueing them via dir_add_name() and finally do add_files(); and - iterate over the index, filtering them by pathspec, and update only the modified/type changed paths but not deleted ones. And "git add -A" becomes exactly the same as above, modulo: - missing $pathspec means "." instead of being an error; and - "iterate over the index" part handles deleted ones as well, i.e. exactly what the current update_callback() in builtin-add.c does. In either case, because fill_directory() does not use read_directory() to read everything in, we need to add an extra logic to iterate over the index to catch mistyped pathspec. Signed-off-by: Junio C Hamano <gitster@pobox.com>