util_git.sh: Checkout to FETCH_HEAD for refspec in "refs/" format

Same reason as "refs/changes" format as explained in commit a9afdbf [1],
all refspec in "refs/" format cannot be directly used to checkout.
So checkout to FETCH_HEAD for all refspec in "refs/" format.

[1]: https://review.trustedfirmware.org/c/ci/tf-m-ci-scripts/+/22012

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I30cb3715165d9e84cc52dd88a88cf1439830ff9b
1 file changed