Visually distinguish files of different levels in the sidebar

[azhuchkov]

What happened?

When a file of upper level appears after files from a subfolder, the sidebar can make the file look like it belongs to the preceding folder.

Steps to reproduce

For example, given this file order:

• subfolder/file.txt
• zzz-root.txt

the sidebar is approximately:

subfolder/
  D file.txt
  M zzz-root.txt

Because root-level files and files inside folder groups use the same
indentation, zzz-root.txt appears to be inside subfolder/.

This can happen because the sidebar preserves the review stream order and
only renders headers for non-root directories.

Expected behavior

Root-level files should be visually distinguishable from files belonging to the preceding folder group.

One possible presentation would be to indent grouped files one level further:

subfolder/
    D file.txt
M zzz-root.txt

Version

0.16.0