Add --include-untracked flag to stash show command

[Golim]

• PR Description

This PR fixes #3931 by adding --include-untracked to git stash show command. Untracked stashed file diff is now shown in the main view.

Before: the untracked stashed file is not shown in lazygit.

After: the untracked stashed file diff is shown in lazygit.

One Issue

This PR does not fix the fact that untracked stashed files are not shown in the list of files in the "Stash" panel when entering a stash entry, I could not find an easy way to make them appear there.

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc