Update Git clients list on empty repository page

Update Git clients list on empty repository page

The page shown for an empty repository lists suggestions for Git clients,
mostly GUI ones, and links to their web pages.

The TortoiseGit client entry is removed since the URL is no longer valid.
The excellent client Fork is added to the closed source clients
section.

This commit also cleans up some other entries, fixing broken tags,
escaping ampersands and removing outdated or wrong information.