bug(github_actions): Skip testing single previous commit

Due to the default of executing bash with `-e -o pipefail` this didn't
fail gracefully anymore. But an explicit check makes more sense anyways.

.github/workflows/main.yml

@@ -343,6 +343,12 @@ jobs:
           git fetch --unshallow --all
 
           rev_list="$(git rev-list --bisect-all origin/${{ github.base_ref }}..fork/${{ github.head_ref }})"
+          # The most recent commit has already been tested. So if that's the
+          # only commit in the PR, we can stop here.
+          if [[ $(echo "$rev_list" | wc -l) -lt 2 ]]; then
+            exit 0
+          fi
+
           commits="$(echo "$rev_list" | grep -v 'dist=0' | cut -d' ' -f 1)"
           n="$(echo "$commits" | wc -l)"