Fix issue 2235. Allow the user to pick Git.exe no matter
where it is installed on their system.
Check ValidSvnWorkindDir before do svn commands. Method GitSvnCommandHelpers.ValidSvnWorkindDir work not correct on submodule repo
Merge branch 'pr/n2236_PhilipDaniels'

Conflicts:
	GitUI/CommandsDialogs/SettingsDialog/CheckSettingsLogic.cs
	GitUI/CommandsDialogs/SettingsDialog/Pages/GitSettingsPage.cs
Merge branch 'pr/n2237_lynxstv'

Conflicts:
	GitCommands/Git/GitSvnCommandHelpers.cs
DownloadExternals.cmd fixed