EDIT: I removed Gemfile.lock from .gitignore, like it said. And now it works.
No Gemfile.lock allowed in .gitignore
Cloning 'https://github.com/discourse/discourse-plugin-skeleton' to '/home/pfaffman/src/discourse-repos/discourse/plugins/discourse-public-bookmark-timers'...
Initializing git repository...
Initialized empty Git repository in /home/pfaffman/src/discourse-repos/discourse/plugins/discourse-public-bookmark-timers/.git/
The following paths are ignored by one of your .gitignore files:
Gemfile.lock
hint: Use -f if you really want to add them.
hint: Turn this message off by running
hint: "git config advice.addIgnoredFile false"
rake aborted!
Command failed with exit 1: git
/home/pfaffman/src/discourse-repos/discourse/lib/tasks/plugin.rake:352:in `system'
/home/pfaffman/src/discourse-repos/discourse/lib/tasks/plugin.rake:352:in `block (2 levels) in <main>'
/home/pfaffman/src/discourse-repos/discourse/lib/tasks/plugin.rake:344:in `chdir'
/home/pfaffman/src/discourse-repos/discourse/lib/tasks/plugin.rake:344:in `block in <main>'
Tasks: TOP => plugin:create
(See full trace by running task with --trace)
I tried again after running git config advice.addIgnoredFile false with the same result.
And also
$ gem exec create-discourse-theme discourse-public-bookmark-timers
ERROR: Could not find a valid gem 'create-discourse-theme' (>= 0) in any repository
ERROR: Possible alternatives: create-discourse-plugin, discourse_theme, askclass-course-theme, date_discreter, opensource-theme