cvsserver: pick up the right mode bits

cvsserver: pick up the right mode bits

When determining the file mode from either ls-tree or diff-tree
output, we used to grab these octal mode string (typically 100644 or
100755) and then did

	$git_perms .= "r" if ( $mode & 4 );
	$git_perms .= "w" if ( $mode & 2 );
	$git_perms .= "x" if ( $mode & 1 );

which was already wrong, as (100644 & 4) is very different from
oct("100644") & 4.  An earlier refactoring 2c3af7e7 (cvsserver:
factor out git-log parsing logic, 2012-10-13) further changed it to
pick the third octal digit (10*0*644 or 10*0*755) from the left and
then do the above conversion, which does not make sense, either.

Let's use the third digit from the last of the octal mode string to
make sure we get the executable and read bits right.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Tested-by: Michael Cronenworth <mike@cchtml.com>