Tab expansion in beta4

[szelpe]

System Details

• posh-git version/path: v1.0.0-beta4
• PowerShell version: tried with both 6.2.5 and 7.0.0
• Git version: 2.26.2.windows.1
• Operating system name and version: Windows 10, Version 2004

Issue Description

I am experiencing a problem with the tab expansion.

I have Set-Alias g git in my $profile and tab completion with g worked in beta2 and beta3, but now broken in beta4. Simply does nothing if I type g fetTab.

Also, if I set the UseLegacyTabExpansion to $True it starts to work again.

[rkeithhill]

It turns out PowerShell's built-in Register-ArgumentCompleter does not take into account aliases to native apps that have registered completers. I think the fix for this is pretty easy. So we will need to pass in any aliases we discover for git,tgit,gitk.

[rkeithhill]

FYI, I'm getting a PR ready to fix this but there is one big caveat. The code to register argument completers for apps and their aliases, happens once during the load of the module. So you will need to ensure your aliases are defined in your profile script before you import posh-git.