export-orig doesn't work when inside a new, temporary merge branch

[Sergio Durigan Junior]

I was doing a merge (update) of exim4 and, after following the steps from <https://wiki.ubuntu.com/UbuntuDevelopment/Merging/GitWorkflow>, I ended up with a new branch called "groovy-exim4-93-15-merge", which is the temporary branch containing the final result of the merge. I then noticed that I didn't have an orig tarball in order to sbuild the package, so I did a "git ubuntu export-orig", and got:

Traceback (most recent call last):
  File "/snap/git-ubuntu/469/bin/git-ubuntu", line 11, in <module>
    load_entry_point('gitubuntu==0.10', 'console_scripts', 'git-ubuntu')()
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/__main__.py", line 254, in main
    sys.exit(args.func(args))
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/exportorig.py", line 120, in cli_main
    tarballs = main(repo, args.commit, orig_search_list)
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/exportorig.py", line 93, in main
    repo.raw_repo.get(commit_hash),
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/build.py", line 1194, in fetch_orig
    tarballs = entry.mechanism(changelog, entry.source)
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/build.py", line 376, in fetch_orig_from_launchpad
    derive_source_from_changelog(changelog),
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/build.py", line 729, in derive_source_from_changelog
    return derive_source_from_series(series)
  File "/snap/git-ubuntu/469/lib/python3.6/site-packages/gitubuntu/source_information.py", line 102, in derive_source_from_series
    raise ValueError("Unable to determine distribution from %s" % series)
ValueError: Unable to determine distribution from groovy

When I switched back to the ubuntu/devel branch, I was then able to properly execute the command above and download the tarball.

I'm using:

$ snap list git-ubuntu
Name Version Rev Tracking Publisher Notes
git-ubuntu 0.10 469 latest/edge canonical✓ classic

on focal.

Thanks!