fsck: use bitwise-or assignment operator to set flag

fsck: use bitwise-or assignment operator to set flag

fsck_tree() has two different ways to set a flag variable, either by
using a if-statement that guards an assignment, or by using a
bitwise-or assignment operator.  Most are done with the former, and
only one variable is assigned with the latter.

Since all the conditions are short-and-sweet, we can afford to
uniformly use the latter style, which makes the resulting code
shorter and easier to read.

Signed-off-by: Hiroyuki Sano <sh19910711@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>