Re-re-re-fix common tail optimization

We need to be extra careful recovering the removed common section, so
that we do not break context nor the changed incomplete line (i.e. the
last line that does not end with LF).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff-interface.c

@@ -121,7 +121,7 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b, long ctx)
 		bp -= blk;
 	}
 
-	while (recovered < trimmed && ctx)
+	while (recovered < trimmed && 0 <= ctx)
 		if (ap[recovered++] == '\n')
 			ctx--;
 	a->size -= (trimmed - recovered);