Fixing ci/prow/check-commit-count.

The implementation of that job is relying on the diff with the `main`
branch.

In order to make sure the `main` branch is up to date, we need to fetch
it first.

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

Author: ybettan

ci/prow/check-commits-count

@@ -2,8 +2,13 @@
 
 set -euxo pipefail
 
-number_of_commits=$(git rev-list --count HEAD ^main)
-if [[ ${number_of_commits} != 1 ]]; then
+git remote add upstream https://github.com/kubernetes-sigs/kernel-module-management.git
+git fetch upstream
+
+number_of_commits=$(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 [[ ${number_of_commits} != 2 ]]; then
     echo '
     All PRs must contain a single commit.
     Please refer to https://github.com/kubernetes-sigs/kernel-module-management/blob/main/CONTRIBUTING.md