Rename `TF_GITLAB_SOURCED` variable to `GITLAB_TOFU_SOURCED`.

This change set renames the `TF_GITLAB_SOURCED` environment variable to
`GITLAB_TOFU_SOURCED` and document the existence of the variable
accordingly in the README.
The reason for renaming it is that the `TF_` variable prefix is "owned"
by OpenTofu / Terraform and not GitLab Tofu. Therefore, we make an
effort to rename these variables to have the `GITLAB_TOFU_` prefix
instead.

Changelog: changed

.gitlab/README.md.template

@@ -300,6 +300,9 @@ and will try to execute a `tofu` command.
 In this case you can set the `GITLAB_TOFU_SOURCE` environment variable
 to `true` before sourcing `gitlab-tofu`.
 
+When the `gitlab-tofu` script is sourced it'll set the `GITLAB_TOFU_SOURCED`
+variable to `true`. This variable is not exported by the script itself.
+
 ### Best Practices
 
 This section is a collection of *some* best practices.

README.md

@@ -315,6 +315,9 @@ and will try to execute a `tofu` command.
 In this case you can set the `GITLAB_TOFU_SOURCE` environment variable
 to `true` before sourcing `gitlab-tofu`.
 
+When the `gitlab-tofu` script is sourced it'll set the `GITLAB_TOFU_SOURCED`
+variable to `true`. This variable is not exported by the script itself.
+
 ### Best Practices
 
 This section is a collection of *some* best practices.

src/gitlab-tofu.sh

@@ -18,6 +18,7 @@ fi
 # Evaluate if this script is being sourced or executed directly.
 # See https://stackoverflow.com/a/28776166
 sourced=0
+# shellcheck disable=SC2153 # it's actually a different variable, thanks shelllcheck.
 if [ "$GITLAB_TOFU_SOURCE" = 'true' ]; then
   sourced=1
 elif [ -n "$ZSH_VERSION" ]; then
@@ -276,5 +277,5 @@ if [ $sourced -eq 0 ]; then
 else
   # This variable can be used if the script is sourced
   # shellcheck disable=SC2034
-  TF_GITLAB_SOURCED=true
+  GITLAB_TOFU_SOURCED=true
 fi

tests/unit/gitlab-tofu.bats

@@ -151,9 +151,9 @@ EOF
 
   cat <<'EOF' > test.sh
 set -x
-test -z "$TF_GITLAB_SOURCED"
+test -z "$GITLAB_TOFU_SOURCED"
 . $(which gitlab-tofu)
-test "$TF_GITLAB_SOURCED"
+test "$GITLAB_TOFU_SOURCED"
 EOF
 
   $SHELL test.sh
@@ -165,9 +165,9 @@ EOF
 
   cat <<'EOF' > test.sh
 set -x
-test -z "$TF_GITLAB_SOURCED"
+test -z "$GITLAB_TOFU_SOURCED"
 . $(which gitlab-tofu)
-test "$TF_GITLAB_SOURCED"
+test "$GITLAB_TOFU_SOURCED"
 EOF
 
   mkdir -p /usr/local/sbin