stash: discard in-process cache after spawning index-changing processes

In 9a67cb4 (stash: convert push to builtin, 2018-12-20), we started
to call `git apply -R --index` and `git reset --hard`, but held onto the
now-stale index in memory. Let's discard it.

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

Author: dscho

builtin/stash.c

@@ -1333,6 +1333,7 @@ static int do_push_stash(struct pathspec ps, const char *stash_msg, int quiet,
 				goto done;
 			}
 		}
+		discard_cache();
 		if (ps.nr) {
 			struct child_process cp_add = CHILD_PROCESS_INIT;
 			struct child_process cp_diff = CHILD_PROCESS_INIT;
@@ -1428,6 +1429,8 @@ static int do_push_stash(struct pathspec ps, const char *stash_msg, int quiet,
 		if (keep_index < 1) {
 			struct child_process cp = CHILD_PROCESS_INIT;
 
+			discard_cache();
+
 			cp.git_cmd = 1;
 			argv_array_pushl(&cp.args, "reset", "-q", "--", NULL);
 			add_pathspecs(&cp.args, ps);