Restore left-arrow shortcut to back out of diff view

[holly-hacker]

This Pull Request fixes/closes #1729.

It changes the following:

• Restore the left-arrow shortcut in the diff view. This is usually a more natural keybind to use when entering the diff view using right-arrow.

I followed the checklist:

• I added unittests
• I ran make check without errors (NOTE: I received newline warnings due to my git config)
• I tested the overall application
• I added an appropriate item to the changelog

[stale]

This issue has been automatically marked as stale because it has not had any activity half a year. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.