Check that git can really be used as well as posh-git

Explicitly call `import posh-git` because it doesn't have a module
manifest thus powershell can't autoload it when using a cmdlet.

At present the cmder repo doesn't try to download posh-git so it might be
missing from the users session.

vendor/profile.ps1

@@ -5,13 +5,22 @@ if( -not $env:PSModulePath.Contains($CmderModulePath) ){
     $env:PSModulePath = $env:PSModulePath.Insert(0, "$CmderModulePath;")
 }
 
+try {
+    Get-command -Name "git" -ErrorAction Stop >$null
+    Import-Module -Name "posh-git" -ErrorAction Stop >$null
+    $gitStatus = $true
+} catch {
+    Write-Warning "Missing git support"
+    $gitStatus = $false
+}
+
 # Set up a Cmder prompt, adding the git prompt parts inside git repos
 function global:prompt {
     $realLASTEXITCODE = $LASTEXITCODE
     $Host.UI.RawUI.ForegroundColor = "White"
     Write-Host $pwd.ProviderPath -NoNewLine -ForegroundColor Green
-    if (Get-Module posh-git) {
         Write-VcsStatus
+    if($gitStatus){
     }
     $global:LASTEXITCODE = $realLASTEXITCODE
     Write-Host "`nλ" -NoNewLine -ForegroundColor "DarkGray"
@@ -19,7 +28,7 @@ function global:prompt {
 }
 
 # Load special features come from posh-git
-if (Get-Module posh-git) {
+if ($gitStatus) {
     Enable-GitColors
     Start-SshAgent -Quiet
 }