ls-files --error-unmatch pathspec error reporting fix.

Earlier patch mistakenly used prefix_len when it meant
prefix_offset.  The latter is to strip the leading directories
when run from a subdirectory.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

ls-files.c

@@ -757,7 +757,7 @@ int main(int argc, const char **argv)
 			if (ps_matched[num])
 				continue;
 			error("pathspec '%s' did not match any.",
-			      pathspec[num] + prefix_len);
+			      pathspec[num] + prefix_offset);
 		}
 		return errors ? 1 : 0;
 	}