files: clear original filename when the user toggles Append or Prepend

When the current filename is equal to the filename of the buffer,
then clear the filename, because the chances that the user wants
to append or prepend a file to itself are practically nil.

This fulfills https://savannah.gnu.org/bugs/?61243.

src/files.c

@@ -2194,9 +2194,13 @@ int do_writeout(bool exiting, bool withprompt)
 			TOGGLE(MAKE_BACKUP);
 			continue;
 		} else if (func == prepend_void) {
+			if (strcmp(answer, openfile->filename) == 0)
+				given[0] = '\0';
 			method = (method == PREPEND) ? OVERWRITE : PREPEND;
 			continue;
 		} else if (func == append_void) {
+			if (strcmp(answer, openfile->filename) == 0)
+				given[0] = '\0';
 			method = (method == APPEND) ? OVERWRITE : APPEND;
 			continue;
 		}