builtin-add.c: restructure the code for maintainability

builtin-add.c: restructure the code for maintainability

The implementation of "git add" has four major codepaths that are mutually
exclusive:

 - if "--interactive" or "--patch" is given, spawn "git add--interactive"
   and exit without doing anything else.  Otherwise things are handled
   internally in this C code;

 - if "--update" is given, update the modified files and exit without
   doing anything else;

 - if "--refresh" is given, do refresh and exit without doing anything
   else;

 - otherwise, find the paths that match pathspecs and stage their
   contents.

It led to an unholy mess in the code structure; each of the latter three
codepaths has a separate call to read_cache(), even though they are all
about "read the current index, update it and write it back", and logically
they should read the index once _anyway_.

This cleans up the latter three cases by introducing a pair of helper
variables:

 - "add_new_files" is set if we need to scan the working tree for paths
   that match the pathspec.  This variable is false for "--update" and
   "--refresh", because they only work on already tracked files.

 - "require_pathspec" is set if the user must give at least one pathspec.
   "--update" does not need it but all the other cases do.

This is in preparation for introducing a new option "--all", that does the
equivalent of "git add -u && git add ." (aka "addremove").

Signed-off-by: Junio C Hamano <gitster@pobox.com>