Add includeUnstaged option to commit slash command

[FozzieHi]

Description

I did debate adding an optional parameter, but thought it was better to be explicit, so that anyone using getDiff in the future can make a decision on whether to include unstaged changes or not. I've therefore updated other uses of getDiff to align with the previous behaviour, but we can change that here if we want to.

Closes #1893.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created