Fix mutation testing on dev branch

[staabm]

shot in the dark, should fix

In Process.php line 112:
                                                                               
  The command "'git' 'diff' 'origin/' '--diff-filter' 'AM' '--name-only' '--'  
   'src'" failed.                                                              
                                                                               
  Exit Code: 128(Invalid exit argument)                                        
                                                                               
  Working directory: /home/runner/work/phpstan-deprecation-rules/phpstan-depr  
  ecation-rules                                                                
                                                                               
  Output:                                                                      
  ================                                                             
                                                                               
                                                                               
  Error Output:                                                                
  ================                                                             
  fatal: bad revision 'origin/'  

[ondrejmirtes]

Thank you!