Updating ci/prow/check-commit-count to use the gh cli.

Prow is cloning the repo using `git clone --mirror ...`.

In addition to that, the changes are directly tested in the `main`
branch, therefore, the current implementation isn't working.

Transitioning to the `gh` cli instead.

Signed-off-by: Yoni Bettan <[email protected]>

Author: ybettan

ci/prow/check-commits-count

@@ -2,13 +2,9 @@
 
 set -euxo pipefail
 
-git remote add upstream https://github.com/kubernetes-sigs/kernel-module-management.git
-git fetch upstream
+commits_count=$(gh pr view ${PULL_NUMBER} -R ${REPO_OWNER}/${REPO_NAME} --json commits --jq '.commits | length')
 
-commits_count=$(git rev-list --count HEAD ^upstream/main)
-# Look like Git (in Prow only) is adding an additional commit for each PR,
-# therefore, we are making sure there are 2 commits instead of 1
-if [[ ${commits_count} != 2 ]]; then
+if [[ ${commits_count} != 1 ]]; then
     echo '
     All PRs must contain a single commit.
     Please refer to https://github.com/kubernetes-sigs/kernel-module-management/blob/main/CONTRIBUTING.md