Run diffs in one go

The flag --diff now runs all the spec files in one run instead of looping through each of them.

Example:

retest --diff main

Setup identified: [RSPEC]. Using command: 'bin/rspec <test>'
Tests found:
  - spec/path/to/file1_spec.rb
  - spec/path/to/file2_spec.rb
  - spec/path/to/file3_spec.rb
Running tests...
Test File Selected: spec/path/to/file1_spec.rb spec/path/to/file2_spec.rb spec/path/to/file3_spec.rb
Running via Spring preloader in process 10493
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Finished in 16.97 seconds (files took 0.69523 seconds to load)
208 examples, 0 failures