Don't use memcpy when source and dest. buffers may overlap

git-index-pack can call memcpy with overlapping source and destination
buffers.  The patch below makes it use memmove instead.

If you want to demonstrate a failure, add the following two lines

+               if (input_offset < input_len)
+                 abort ();

before the existing memcpy call (shown in the patch below),
and then run this:

  (cd t; sh ./t5500-fetch-pack.sh)

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

index-pack.c

@@ -96,7 +96,7 @@ static void flush(void)
 		if (output_fd >= 0)
 			write_or_die(output_fd, input_buffer, input_offset);
 		SHA1_Update(&input_ctx, input_buffer, input_offset);
-		memcpy(input_buffer, input_buffer + input_offset, input_len);
+		memmove(input_buffer, input_buffer + input_offset, input_len);
 		input_offset = 0;
 	}
 }