Log: add "Repositories" view [SG-11649]

See also: https://smartgit.userecho.com/topics/655

TL;DR: If the Log window had a Repositories view, switching between repos is as easy as on the main window, and does not require finding/launching and switching to the main window to do so.

I understand the design decision to have separate windows for the different macro activities, so this request is an idea that will help a lot with usability in that design and is possibly a reasonable answer for those that want a single window.

I often work on large products and they often have over 25 repositories; this causes a lot of switching between repos.  I use one main window and easily switch between repos; occasionally I open a second or third main window but not usually.

I regularly open a log window when needed on a repo to review commits/diffs.  This is the usability problem - having to find the main window, select the repo, and open the Log window.