Compare browser lists entries that exist only in the compared repository after all the base repository's entries, out of name order #503

What's wrong

Since #482, RepoBrowsing.List returns a folder's entries "in name order ignoring case", so that "the browsers read the same on every machine" (ProjectDirector/RepoBrowsing.cs:26-36).

The compare browser throws that order away when it merges the two sides:

IEnumerable<RelativePath> allFilesystemEntries = BrowserContentsBase.Union(BrowserContentsCompare);   // ProjectDirector.cs:2076
...
Collection<RelativePath> files = allFilesystemEntries.Except(directories).ToCollection();             // :2080

Enumerable.Union yields every distinct element of the first sequence and only then the new elements of the second. So every entry that exists only in the compared repository is appended after all the base entries, in both the folder group and the file group.

Failure scenario

The base folder holds a.txt and c.txt, and the compared folder holds b.txt and c.txt. The compare browser shows a.txt, c.txt, b.txt; checked with new[]{"a.txt","c.txt"}.Union(new[]{"b.txt","c.txt"}).

The rows that end up out of place are exactly the compared-only ones, which are the rows offering "Take". In a large folder they pile up at the bottom of each group instead of sitting next to their neighbours. The single-repo browser is not affected.

Suggested fix / acceptance criteria

• Sort the merged listing by the rule List uses, e.g. .OrderBy(e => e.WeakString, StringComparer.OrdinalIgnoreCase). Doing it once in SwitchCompareBrowserPath also fits Repo and compare browsers stat every entry on the file system on every frame (plus O(n²) Contains lookups) #498's plan to cache the listing.
• Pull the merge into a plain method (e.g. RepoBrowsing.Merge(base, compare)) and add a RepoBrowsingTests case: base [a.txt, c.txt] plus compare [b.txt, c.txt] gives [a.txt, b.txt, c.txt], each entry once, with folders and files each in name order.