Refresh index without discard_index + repo_read_index

[Roman Perepelitsa <roman.perepelitsa@gmail.com>]

I’m writing a program that reads stdin in a loop and prints the
equivalent of `git status` whenever it reads a character. It always
prints its results for the same repository, the same pathspec, etc.
The input characters have no effect on the output, only the sate of
the repository does.
I’m hoping that I can make it produce results faster than a bash
script that literally calls `git status` in a loop. I’m thinking that
maybe I can keep some caches around so that I don’t have to redo all
the work on every iteration that git status does.

What I cannot figure out is how to refresh the index so that it picks
up all the changes that might have happened to the repository and
workdir since the last iteration. Here’s what I have:

```c
#include <stdio.h>
#include <string.h>

#define USE_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "wt-status.h"
#include "pathspec.h"
#include "repository.h"

int cmd_multi_status(int argc, const char** argv, const char* prefix) {
  struct pathspec pathspec;
  memset(&pathspec, 0, sizeof(pathspec));
  repo_read_index(the_repository);
  while (getchar() != EOF) {
    refresh_index(&the_index,
                  REFRESH_QUIET | REFRESH_UNMERGED | REFRESH_REALLY,
                  &pathspec, NULL, NULL);
    int uncommitted = has_uncommitted_changes(the_repository, 1);
    int unstaged = has_unstaged_changes(the_repository, 1);
    int untracked = has_untracked(the_repository);  // I added this to
wt-status.h
    printf("%d %d %d\n", uncommitted, unstaged, untracked);
  }
  return 0;
}
```

This produces correct results on the first iteration but then it
doesn’t pick up all changes. I’ve hacked some code in preload-index.c
and cache.c so that it picks up more changes but I still cannot detect
when an unstaged file becomes uncommitted or the other way around.

Any pointers would be greatly appreciated.

Roman.

P.S.

I have working code that uses libgit2. Updating the index with
`git_index_read(index, 0)` works as expected and is indeed faster than
discarding the index and starting from scratch. But it's still slower
than `git status` on large repositories, especially with many ignored
files.