Indent first level of subtrees in the sidebar

[FichteFoll]

Problem description

The current way to show tree structures in the sidebar is quite unfortunate because the first level of each section is effectively indented by the same margin as the section headings themselves, which means the disclosure arrow to expand or collapse the tree is in the same column as those from the sections. This is quite confusing, as supported by other users on Discord. In my opinion, the column at which the first character begins is the most significant indicator of the level (through indentation). Applies to both BRANCHES and REMOTES sections.

Preferred solution

Just indent the first level by one.

Alternatives

Only indent the first level if there is a subtree to expand. The downside is that in the REMOTES section you will almost always have a subtree, so they will still not align with the others from TAGS and SUBMODULES (and BRANCHES potentially).

Screenshots

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[23:14] wbond: I believe the issue is mostly that layouts that look right with "nested" branches and lots of remotes/branches tend to look off with simpler repos. Not that we can't improve it, but it isn't necessarily super clear. For example, indenting branches enough to have a disclosure arrow when a user has nested branch names looks really badly indented when a user has no nested branches.

[FichteFoll]

Here's another instance of this where it took me several seconds to find the upmaster branch in the tree that I wanted to checkout. Maybe it would also be a good idea to dim "folders" that aren't actual branches?