commit --interactive: make it work with the built-in `add -i`

The built-in `git add -i` machinery obviously has its `the_repository`
structure initialized at the point where `cmd_commit()` calls it, and
therefore does not look at the environment variable `GIT_INDEX_FILE`.

But it has to, because the index was already locked, and we want to ask
the interactive add machinery to work on the `index.lock` file instead
of the `index` file.

Technically, we could teach `run_add_i()` (and `run_add_p()`) to look
specifically at that environment variable, but the entire idea of
passing in a parameter of type `struct repository *` is to allow working
on multiple repositories (and their index files) independently.

So let's instead override the `index_file` field of that structure
temporarily.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/commit.c

@@ -344,7 +344,7 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 		die(_("index file corrupt"));
 
 	if (interactive) {
-		char *old_index_env = NULL;
+		char *old_index_env = NULL, *old_repo_index_file;
 		hold_locked_index(&index_lock, LOCK_DIE_ON_ERROR);
 
 		refresh_cache_or_die(refresh_flags);
@@ -352,13 +352,17 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 		if (write_locked_index(&the_index, &index_lock, 0))
 			die(_("unable to create temporary index"));
 
+		old_repo_index_file = the_repository->index_file;
+		the_repository->index_file =
+			(char *)get_lock_file_path(&index_lock);
 		old_index_env = xstrdup_or_null(getenv(INDEX_ENVIRONMENT));
-		setenv(INDEX_ENVIRONMENT, get_lock_file_path(&index_lock), 1);
+		setenv(INDEX_ENVIRONMENT, the_repository->index_file, 1);
 
 		if (interactive_add(argc, argv, prefix,
 				    patch_interactive ? "" : NULL) != 0)
 			die(_("interactive add failed"));
 
+		the_repository->index_file = old_repo_index_file;
 		if (old_index_env && *old_index_env)
 			setenv(INDEX_ENVIRONMENT, old_index_env, 1);
 		else