ci: fix issues with missing base commit

Fix issues like: `git: base SHA1 (0000000000000000000000000000000000000000) doesn't exist. Make sure that the base branch is up-to-date.`

Author: jamacku

.github/workflows/differential-shellcheck.yml

@@ -4,6 +4,7 @@ name: Differential ShellCheck
 
 on:
   push:
+    branches: [ main, rhel*-branch ]
   pull_request:
     branches: [ main, rhel*-branch ]