Don't allow submit in the middle of a rebase

[mckaydavis]

Summary

This PR makes stack-pr submit exit early if the tree is in the middle of a git rebase operation.

Problem

stack-pr submit on a repository that is in the middle of a rebase will partially complete then error out.

Context: I had used git rebase to modify some old commits and forgot to do the final git rebase --continue. I then then ran stack-pr submit. It will look like it is working until erroring out with a non-helpful traceback:

Updating commit messages with stack metadata
Updating remote branches
Adding cross-links to PRs
Rebasing the original branch 'HEAD'
Exitcode: 128
Stdout: None
Stderr: None
Traceback (most recent call last):
  File "/Users/mdavis/git/main/.derived/autovenv/exposed-bin/stack-pr", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/mdavis/git/main/.derived/autovenv/lib/python3.11/site-packages/stack_pr/cli.py", line 1401, in main
    command_submit(
  File "/Users/mdavis/git/main/.derived/autovenv/lib/python3.11/site-packages/stack_pr/cli.py", line 951, in command_submit
    run_shell_command(
  File "/Users/mdavis/git/main/.derived/autovenv/lib/python3.11/site-packages/stack_pr/shell_commands.py", line 42, in run_shell_command
    return subprocess.run(list(map(str, cmd)), **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['git', 'rebase', 'mckaydavis/stack/15', 'HEAD', '--committer-date-is-author-date']' returned non-zero exit status 128.

[atomicapple0]

Thank you! Looks clean.