Add user config for hiding the root item in the file tree

[stefanhaller]

• PR Description

In #4346 we added a / root item in the Files and CommitFiles panels whenever there is more than one top-level item. We made it unconditional, but I promised to add a config as soon as users ask for being able to disable it. For a while I was able to convince users who asked for it that it is useful and they don't want to turn it off, but now there's a stronger request from someone who refuses to upgrade to the current version, and we don't want that.

So, add a config option gui.showRootItemInFileTree that is true by default.

• 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

[stefanhaller]

I wasn't sure if we want this dependency. It was the easiest way to implement this; without the dependency, we'd have to pass the showRootItem bool all the way through all the call chains that end up calling something like SetTree, and these call chains are long (starting from a class that knows the userConfig), so that would be a lot of changes. Let me know what you think @jesseduffield

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for da32b591	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (da32b59)	Report Missing	Report Missing	Report Missing
Head commit (3cff484)	56484	49062	86.86%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4593)	57	57	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[jesseduffield]

LGTM. I'm okay with having that dependency