I have a new branch named `test_branch` containing more than one commit and I want to merge it to the `master` branch. Before merging I need to get the difference between `test_branch` and `master` branch with GitLab CI.

I tried to use the command below but it failed:

```
# the variable $CI_COMMIT_MESSAGE is "Merge branch 'test_branch' into 'master'" so I can get the new branch name
git diff $(echo $CI_COMMIT_MESSAGE | awk -F "'" '{print $2}') master --name-status

```

and shows:

```
fatal: ambiguous argument 'test_branch': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

```

Why is it failed and what is the correct way doing this?