Update shell_history.cpp to account for .ash_history files

To submit a PR please make sure to follow the next steps:

• Read the CONTRIBUTING.md guide at the root of the repo.
• Ensure the code is formatted building the format_check target.
  If it is not, then move the committed files to the git staging area,
  build the format target to format them, and then re-commit.
  More information is available on the wiki.
• Ensure your PR contains a single logical change.
• Ensure your PR contains tests for the changes you're submitting.
• Describe your changes with as much detail as you can.
• Link any issues this PR is related to.
• Remove the text above.