Add worktree option to fast forwarding operation

[john-mutuma]

• PR Description

Fix a reported issue regarding Fast-forwarding a branch that is checked out in a different worktree. It's reported, and I could repro, that whenever such an action is taken, the current worktree is polluted with unwanted File changes related to the Fast-forward operation.

A solution is suggested – and tested to produce expected results – that adding --work-tree option to the generated command should fix the issue.

Issue: 2957

I'm proposing to merge these changes as it produces expected results:

• Please check if the PR fulfills these requirements

• 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

[jesseduffield]

Thanks for raising this. I'd copy+paste this test and tweak it for this PR.