Create new item display handling for uncommitted folder in source control pane

[juliasilge]

In #1374 we noted how some common data science workflows result in a huge explosion of files in the git pane. It is hard to navigate the source control pane when this happens, and it feels quite overwhelming, especially compared to the RStudio experience where folders are collapsed by default and allow committing the entire folder pretty easily.

We tried switching the default for the source control pane from "list" to "tree" but no longer think this is a good solution; we reverted the change in #1553. Instead, let's create a special item type for the source control pane display for an uncommitted folder, with a "+" to add the whole folder; this item type would work in both "list" and "tree" mode. This is more work than only switching the default display type, but it will solve the problem of navigating the source control pane in a better way.