remote.c: don't leak the base branch name in format_tracking_info

Found by scan.coverity.com (Id: 1127809)

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -1925,7 +1925,7 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 int format_tracking_info(struct branch *branch, struct strbuf *sb)
 {
 	int ours, theirs;
-	const char *base;
+	char *base;
 	int upstream_is_gone = 0;
 
 	switch (stat_tracking_info(branch, &ours, &theirs)) {
@@ -1941,8 +1941,7 @@ int format_tracking_info(struct branch *branch, struct strbuf *sb)
 		break;
 	}
 
-	base = branch->merge[0]->dst;
-	base = shorten_unambiguous_ref(base, 0);
+	base = shorten_unambiguous_ref(branch->merge[0]->dst, 0);
 	if (upstream_is_gone) {
 		strbuf_addf(sb,
 			_("Your branch is based on '%s', but the upstream is gone.\n"),
@@ -1988,6 +1987,7 @@ int format_tracking_info(struct branch *branch, struct strbuf *sb)
 			strbuf_addf(sb,
 				_("  (use \"git pull\" to merge the remote branch into yours)\n"));
 	}
+	free(base);
 	return 1;
 }