{reset,merge}: call discard_index() before returning

These two built-ins both deal with the index, but weren't discarding
it. In subsequent commits we'll add more free()-ing to discard_index()
that we've missed, but let's first call the existing function.

We can doubtless add discard_index() (or its alias discard_cache()) to
a lot more places, but let's just add it here for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/merge.c

@@ -1794,5 +1794,6 @@ done:
 	}
 	strbuf_release(&buf);
 	free(branch_to_free);
+	discard_index(&the_index);
 	return ret;
 }

builtin/reset.c

@@ -481,5 +481,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (!pathspec.nr)
 		remove_branch_state(the_repository, 0);
 
+	discard_index(&the_index);
+
 	return update_ref_status;
 }