What is the keyboard shortcut for 'Commit' in the 'commit changes' dialogue in RubyMine (and PHPStorm, IntellijIDEA, PyCharm etc)?

Mnemonics usually work via the Alt key, so it would be Alt+I.

Note that Enter confirms the dialog choice and if the focus is in the text area, it would be Ctrl+Enter.

Alt+I mnemonic will also not work when you are entering the commit comment, so you should use Ctrl+Enter.

You can also try disabling this option to get separate buttons.