builtin/diff: fix botched update of usage comment

In the previous commit, an attempt was made to correct the "N=1, M=0"
case. However, the fix was botched and it introduced two half-correct
sections by mistake. Combine these half-correct sections into one fully
correct section.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Denton-L

builtin/diff.c

@@ -284,11 +284,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	 * N=1, M=0:
 	 *      tree vs files (diff-index)
 	 *
-	 * N=1, M=0:
-	 *      tree vs cache (diff-index --cached)
-	 *
 	 * N=1, M=0, --cached:
-	 *      tree vs files (diff-index)
+	 *      tree vs cache (diff-index --cached)
 	 *
 	 * N=2, M=0:
 	 *      tree vs tree (diff-tree)