files: let ^C cancel the exiting when the file on disk was changed

When the user exits with ^X and gets warned that the file on disk
has changed, then typing ^C at the question whether to continue
saving should not discard the buffer and exit, but should return
the user to the filename prompt.

This fixes https://savannah.gnu.org/bugs/?61883.
Reported-by: Tasos Papastylianou <tpapastylianou@hotmail.com>

Bug existed since version 2.9.0, commit 217cfbf3.

src/files.c

@@ -2306,6 +2306,8 @@ int write_it_out(bool exiting, bool withprompt)
 						return 2;
 					else
 						return 0;
+				} else if (choice < 0 && exiting) {
+					continue;
 				} else if (choice != 1) {
 					free(given);
 					return 1;