CommandNotFoundAction FTW!

ReplaceGitAlias.ps1

$gitAliases = 
    (git config --global -l).Where{ 
            $_ -match '^alias\.'
        }.ForEach{
            $_ -replace '^alias\.(\w+).*', '$1'
        }

$ExecutionContext.InvokeCommand.CommandNotFoundAction = { 
    param ($name, $eventArgs) 
    if ($name -in $gitAliases) { 
        $alias = $name
    } elseif ($aliases = $gitAliases -match [regex]::Escape($name)) { 
        $alias = $aliases | 
            Sort-Object -Property Length  | 
            Select-Object -First 1 
    } 
    
    if ($alias) { 
        $eventArgs.CommandScriptBlock = { git $alias @args }.GetNewClosure()
    } 
}