[Buildkite] Fix docs bypass for origin/master divergences (#764)

If origin/master diverges beyond the initial fork-point on branches or external forks, the `git merge-base` command exits unsuccessfully. This will cause commits to incorrectly be recognised as a docs bypass. This change will catch the unsuccessful exit and treat it as a normal CI/CD run.

Examples:
- https://buildkite.com/authelia/authelia/builds/932
- https://buildkite.com/authelia/authelia/builds/933
- https://buildkite.com/authelia/authelia/builds/941

.buildkite/deployment.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
 set -u
 
+DIVERGED=$(git merge-base --fork-point origin/master > /dev/null; echo $?)
+
+if [[ $DIVERGED -eq 0 ]]; then
   if [[ $BUILDKITE_TAG == "" ]]; then
     if [[ $BUILDKITE_BRANCH == "master" ]]; then
       CI_DOCS_BYPASS=$(git diff --name-only HEAD~1 | sed -rn '/^docs\/.*/!{q1}' && echo true || echo false)
@@ -10,6 +13,9 @@ if [[ $BUILDKITE_TAG == "" ]]; then
   else
     CI_DOCS_BYPASS="false"
   fi
+else
+  CI_DOCS_BYPASS="false"
+fi
 
 cat << EOF
 env:

.buildkite/pipeline.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
 set -u
 
+DIVERGED=$(git merge-base --fork-point origin/master > /dev/null; echo $?)
+
+if [[ $DIVERGED -eq 0 ]]; then
   if [[ $BUILDKITE_TAG == "" ]]; then
     if [[ $BUILDKITE_BRANCH == "master" ]]; then
       CI_DOCS_BYPASS=$(git diff --name-only HEAD~1 | sed -rn '/^docs\/.*/!{q1}' && echo true || echo false)
@@ -14,6 +17,9 @@ if [[ $BUILDKITE_TAG == "" ]]; then
   else
     CI_DOCS_BYPASS="false"
   fi
+else
+  CI_DOCS_BYPASS="false"
+fi
 
 cat << EOF
 env: