Merging pull request throwing 500 internal server error: bad revision

[hanhpv]

• Gitea version (or commit ref): 1.9.3
• Git version: 1.8.3.1
• Operating system: CentOS 7.6
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Description

Steps to reproduce:

• create a new branch
• push branch
• create a new pull request, it will be available to be merged automatically according to Gitea
• clicking the "Merge Pull Request" button results in a 500 error

Log message:

2019/09/12 19:48:00 routers/repo/pull.go:629:MergePullRequest() [E] Merge: getDiffTree: git diff-tree [/var/lib/gitea/data/tmp/local-repo/merge-664547743.git base:master head:head_repo/deploy_fi]: fatal: bad revision 'head_repo/deploy_fi'

Screenshots

[hanhpv]

Duplicated of #8133