chore(ci): fix conventional commit check

[bshaffer]

Add origin/ in order to fix unexpected error in the conventional commit check:

In execute.php line 166:
                                                                               
  Shell command "exec git 'rev-parse' 'owl-bot-copy-DocumentAi'" returned an   
  exit code of "128".                                                          
                                                                               
  STDOUT:                                                                      
      owl-bot-copy-DocumentAi                                                  
                                                                               
                                                                               
  STDERR:                                                                      
      fatal: ambiguous argument 'owl-bot-copy-DocumentAi': unknown revision o  
  r path not in the working tree.                                              
      Use '--' to separate paths from revisions, like this:                    
      'git <command> [<revision>...] -- [<file>...]'                           
                                                                               

roave-backwards-compatibility-check:assert-backwards-compatible [--from [FROM]] [--to TO] [--format [FORMAT]] [--install-development-dependencies]

We need to test this fix on an owlbot PR before merging

[bshaffer]

Tested against #7765, and it ran as expected!

https://github.com/googleapis/google-cloud-php/actions/runs/11580353226/job/32238669708?pr=7765#step:5:309