kernel-yocto: use cat-file instead of git-show

kernel-yocto: use cat-file instead of git-show

Parsing the output of git show is error prone, since it changes based on
the type of issue with bad comit IDs. Since the output is no longer used
in the case of a valid ref, we can switch to git-cat-file and  simply
check the return code.

(From OE-Core rev: 228c05013fe691321ec00467d8d0c0bb64dd175c)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>