fix(git): tighten up scripts to avoid inconsistencies

* E.g. Minimise issues with new branches not being created from the
  latest commit in the `upstream` branch, even after pulling
* It's also a likelihood that issues are triggered by branch names
  coming through in the YAML; I've needed quoting in the past when numbers
  are involved -- probably best to tighten up strings with `yamllint`

ssf/files/default/git/git_10_prepare.sh

@@ -14,33 +14,34 @@ COMMENT='Command `'${STATE}'` run'
 # Check if PR branch already exists
 BRANCH=$(git branch -l "${BRANCH_PR}")
 COMMIT=
-if [ "${BRANCH}" ]; then
+if [ ! -z "${BRANCH}" ]; then
     git checkout ${BRANCH_PR}
     # This may end up as blank as well, so that's why a separate `if` is required below
     COMMIT=$(git log -n1 | grep "${COMMIT_GREP}")
 fi
 
 # Perform actions depending on if a commit was found or not
-if [ "${COMMIT}" ]; then
+if [ ! -z "${COMMIT}" ]; then
     CHANGED=False
 else
     git checkout ${BRANCH_UPSTREAM}
     git pull
+    git status
     # If the branch existed but not the commit, assume the branch is stale (i.e. previous PR merged)
     # Remove it, ready to be recreated at the latest upstream commit
-    if [ "${BRANCH}" ]; then
+    if [ ! -z "${BRANCH}" ]; then
         git branch -d ${BRANCH_PR}
     fi
     # TODO: Improve this part, should be able to remove with the duplication with the right solution
     # Don't want to resort to using `git branch -D` above, since that could be premature in certain situations
     # Branch may _still_ exist since it might not be merged upstream
     BRANCH=$(git branch -l "${BRANCH_PR}")
-    if [ "${BRANCH}" ]; then
-        git checkout ${BRANCH_PR}
-        git merge ${BRANCH_UPSTREAM}
-    else
-        git checkout -b ${BRANCH_PR}
+    NEW_BRANCH=''
+    if [ -z "${BRANCH}" ]; then
+        NEW_BRANCH='-b'
     fi
+    git checkout ${NEW_BRANCH} ${BRANCH_PR}
+    git merge ${BRANCH_UPSTREAM}
 fi
 
 # Write the state line

ssf/files/default/git/git_20_commit_push.sh

@@ -22,7 +22,7 @@ COMMENT='Command `'${STATE}'` run'
 
 # Prepare git options depending on if a commit was found or not
 COMMIT=$(git log -n1 | grep "${COMMIT_GREP}")
-if [ "${COMMIT}" ]; then
+if [ ! -z "${COMMIT}" ]; then
     AMEND='--amend'
     FORCE='-f'
 else

ssf/files/default/git/git_30_create_PR.sh

@@ -23,7 +23,7 @@ COMMENT='Command `'${STATE}'` run'
 # Only create the PR if it doesn't already exist
 # If it already exists, the `git push` done earlier will have updated the PR already
 PR_EXISTS=$(curl -i https://api.github.com/repos/${REPO_OWNER}/${REPO_NAME}/pulls | grep "${GH_USER}:${BRANCH_PR}")
-if [ "${PR_EXISTS}" ]; then
+if [ ! -z "${PR_EXISTS}" ]; then
     CHANGED=False
 else
     curl -H "Authorization: bearer ${GH_TOKEN}" -d '