git ubuntu submit ... --branch fails

[Christian Ehrhardt ]

I have not found a way to add --branch in a working way.

Use-Case:
I have three branches (Bionc/Cosmic/Disco) that I want to push merges for
I wanted to use "git-ubuntu submit ... --branch" to be able to do so without needing to checkout in between.

But being on branch "lp-1828771-CAP_AUDIT_WRITE-disco" I issue this and fail:
$ git ubuntu submit --reviewer canonical-server-packageset-reviewers --target-branch ubuntu/bionic-devel --branch lp-1828771-CAP_AUDIT_WRITE-bionic
Traceback (most recent call last):
  File "/snap/git-ubuntu/451/bin/git-ubuntu", line 11, in <module>
    load_entry_point('gitubuntu==0.7.4', 'console_scripts', 'git-ubuntu')()
  File "/snap/git-ubuntu/451/lib/python3.6/site-packages/gitubuntu/__main__.py", line 243, in main
    sys.exit(args.func(args))
  File "/snap/git-ubuntu/451/lib/python3.6/site-packages/gitubuntu/submit.py", line 89, in cli_main
    proto=args.proto,
  File "/snap/git-ubuntu/451/lib/python3.6/site-packages/gitubuntu/submit.py", line 143, in main
    pkgname = repo.get_changelog_srcpkg_from_treeish(commitish_string)
UnboundLocalError: local variable 'commitish_string' referenced before assignment

If I check out "lp-1828771-CAP_AUDIT_WRITE-bionic" and run withut --branch it works:
$ git checkout lp-1828771-CAP_AUDIT_WRITE-bionic
Switched to branch 'lp-1828771-CAP_AUDIT_WRITE-bionic'
$ git ubuntu submit --reviewer canonical-server-packageset-reviewers --target-branch ubuntu/bionic-devel
Your merge proposal is now available at: https://code.launchpad.net/~paelzer/ubuntu/+source/openvpn/+git/openvpn/+merge/367393
If it looks ok, please move it to the 'Needs Review' state.