Gitlab merge request are broken

[koushyk]

Checklist

• I've searched the issue queue to verify this is not a duplicate bug report.
• I've included steps to reproduce the bug.
• I've pasted the output of kargo version.
• I've pasted logs, if applicable.

Description

When promoting a fright to stage which has merge request creation enabled I'm getting an error

error executing Git-based promotion mechanisms: error executing Kargo Render promotion mechanism: error preparing PR branch "https://<gitlab_url>/deploy/service.git": error checking out branch "master\n" from repo "https://<gitlab_url>//deploy/service.git": error executing cmd [/usr/bin/git checkout master --]: fatal: invalid reference: master

but for stages without merge request features promotions are working as they should.

Screenshots

Steps to Reproduce

1. Create a warehouse with helm and image in different repositories.
2. Create stage

kind: Warehouse
spec:
  freightCreationPolicy: Automatic
  interval: 5m0s
  subscriptions:
    - git:
        branch: master
        commitSelectionStrategy: NewestFromBranch
        discoveryLimit: 20
        includePaths:
          - service
        repoURL: https://<gitlab_url>/service.git
        strictSemvers: true
    - image:
        allowTags: ^[^sha256].*$
        discoveryLimit: 20
        gitRepoURL: https://<gitlab_url>/service.git
        imageSelectionStrategy: NewestBuild
        repoURL: <registry_url>/service
        strictSemvers: true
---
apiVersion: kargo.akuity.io/v1alpha1
kind: Stage
metadata:
  name: <stage-name>
spec:
  promotionMechanisms:
    argoCDAppUpdates:
      - appName: <argocd_app_name>
    gitRepoUpdates:
      - render: {}
        repoURL: https://<gitlab_url>/service.git
        writeBranch: env/<env-name>/<service-name>
  requestedFreight:
    - origin:
        kind: Warehouse
        name: <service-name>-wh
      sources:
        direct: true
  shard: stage
---
 apiVersion: kargo.akuity.io/v1alpha1 
 kind: Stage
metadata:
  name: <stage-name>
spec:
  promotionMechanisms:
    gitRepoUpdates:
      - render: {}
        repoURL: https://<gitlab_url>/deploy/service.git
        writeBranch: env/<env>/<service-name>
        pullRequest:
          gitlab: {}
  requestedFreight:
    - origin:
        kind: Warehouse
        name: <service-name>-wh
      sources:
        stages:
          - <stage-name>

4. Promote fright

Version

{
  "Version": "v0.9.1",
  "BuildDate": "2024-10-11T04:59:22Z",
  "GitCommit": "a1a43b7c59780e726b6438fd41cc74597dd442b6",
  "GitTreeDirty": false,
  "GoVersion": "go1.23.2",
  "Compiler": "gc",
  "Platform": "linux/amd64"
}

Logs

Paste any relevant application logs here.

[krancour]

@koushyk is <gitlab_url> on gitlab.com or self-hosted GitLab?

[koushyk]

@krancour self-hosted