built-in add -p: implement the "stash" and "reset" patch modes

The `git stash` and `git reset` commands support a `--patch` option, and
both simply hand off to `git add -p` to perform that work. Let's teach
the built-in version of that command to be able to perform that work, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.h

@@ -25,6 +25,8 @@ int run_add_i(struct repository *r, const struct pathspec *ps);
 
 enum add_p_mode {
 	ADD_P_ADD,
+	ADD_P_STASH,
+	ADD_P_RESET,
 };
 
 int run_add_p(struct repository *r, enum add_p_mode mode,