Fix a bug with replacing $USERNAME in the branch name. (#44)

Author: ZolotukhinM

src/stack_pr/cli.py

@@ -567,9 +567,7 @@ def get_branch_name_base(branch_name_template: str):
     username = get_gh_username()
     current_branch_name = get_current_branch_name()
     branch_name_base = branch_name_template.replace("$USERNAME", username)
-    branch_name_base = branch_name_template.replace(
-        "$BRANCH", current_branch_name
-    )
+    branch_name_base = branch_name_base.replace("$BRANCH", current_branch_name)
     return branch_name_base