Add GitPromptValues global to allow error status display in prompt

It is useful to display the error status of the last command.  But in
order to do that we must preserve the value of $? and make the
original $LASTEXITCODE available to string expansion in the prompt.

README.md

@@ -375,12 +375,46 @@ Prompt layout when DefaultPromptWriteStatusFirst is set to $true:
 {DPPrefix}<{BeforeStatus}{Status}{AfterStatus}>{PathStatusSeparator}{DPPath}{DPBeforeSuffix}<{DPDebug}><{DPTimingFormat}>{DPSuffix}
 ```
 
+If you want to display the error status of the last command, you can use the values stored in the `$global:GitPromptValues`
+object which includes the value of `$LastExitCode` and `$?` (represented by the property `DollarQuestion`). Here is
+a prompt customization that displays a Red exit code value when `$LastExitCode` is non-zero or a Red `!` if `$?`
+is `$false`:
+
+```powershell
+function global:PromptWriteErrorInfo() {
+    if ($global:GitPromptValues.DollarQuestion) { return }
+
+    if ($global:GitPromptValues.LastExitCode) {
+        "`e[31m(" + $global:GitPromptValues.LastExitCode + ") `e[0m"
+    }
+    else {
+        "`e[31m! `e[0m"
+    }
+}
+
+$global:GitPromptSettings.DefaultPromptBeforeSuffix.Text = '`n$(PromptWriteErrorInfo)$([DateTime]::now.ToString("MM-dd HH:mm:ss"))'
+```
+
+When a PowerShell command fails, this is the prompt you will see:
+
+![~\GitHub\posh-git [master ≡]&#10;! 07-01 22:36:31> ][prompt-error1]
+
+When an external application returns a non-zero exit code, 1 in this case, you will see the exit code in the prompt:
+
+![~\GitHub\posh-git [master ≡]&#10;(1) 07-01 22:32:28> ][prompt-error2]
+
+Note that until you run an external application that resets `$LASTEXITCODE` to zero, you will see the exit code for
+any error.  In addition to `LastExitCode` and `DollarQuestion`, `$global:GitPromtpValues` also has `IsAdmin` and
+`LastPrompt` properties.  The `LastPrompt` property contains the ANSI escaped string that was used for the last prompt.
+This can be useful for debugging your prompt display particular when using ANSI/VT sequences.
+
 If you require even more customization than `$GitPromptSettings` provides, you can create your own prompt
 function to show whatever information you want.
 See the [Customizing Your PowerShell Prompt][wiki-custom-prompt] wiki page for details.
 
-However, if you need a custom prompt to perform some non-prompt logic, you can still use posh-git's prompt function to
-write out a prompt string.  This can be done with the `$GitPromptScriptBlock` variable as shown below e.g.:
+However, if you need a custom prompt just in order to perform some non-prompt logic, you can still use posh-git's
+prompt function to write out a prompt string.  This can be done with the `$GitPromptScriptBlock` variable as shown
+below e.g.:
 
 ```powershell
 # my profile.ps1
@@ -448,6 +482,8 @@ function prompt {
 [prompt-swap]:     https://github.com/dahlbyk/posh-git/wiki/images/PromptStatusFirst.png   "[master ≡] ~\GitHub\posh-git> "
 [prompt-two-line]: https://github.com/dahlbyk/posh-git/wiki/images/PromptTwoLine.png       "~\GitHub\posh-git [master ≡]&#10;> "
 [prompt-custom]:   https://github.com/dahlbyk/posh-git/wiki/images/PromptCustom.png        "[master ≡] ~\GitHub\posh-git&#10;02-18 14:04:35 38> "
+[prompt-error1]:   https://github.com/dahlbyk/posh-git/wiki/images/PromptError1.png        "~\GitHub\posh-git [master ≡]&#10;! 07-01 22:36:31> "
+[prompt-error2]:   https://github.com/dahlbyk/posh-git/wiki/images/PromptError2.png        "~\GitHub\posh-git [master ≡]&#10;(1) 07-01 22:32:28> "
 
 [v0-change]:       https://github.com/dahlbyk/posh-git/blob/v0/CHANGELOG.md
 [v0-readme]:       https://github.com/dahlbyk/posh-git/blob/v0/README.md

src/GitPrompt.ps1

@@ -2,6 +2,7 @@
 # http://www.markembling.info/view/my-ideal-powershell-prompt-with-git-integration
 
 $global:GitPromptSettings = [PoshGitPromptSettings]::new()
+$global:GitPromptValues = [PoshGitPromptValues]::new()
 
 # Override some of the normal colors if the background color is set to the default DarkMagenta.
 $s = $global:GitPromptSettings

src/PoshGitTypes.ps1

@@ -292,3 +292,10 @@ class PoshGitPromptSettings {
 
     [bool]$Debug = $false
 }
+
+class PoshGitPromptValues {
+    [int]$LastExitCode
+    [bool]$DollarQuestion
+    [bool]$IsAdmin
+    [string]$LastPrompt
+}

src/posh-git.psm1

@@ -29,6 +29,10 @@ else {
 
 # The built-in posh-git prompt function in ScriptBlock form.
 $GitPromptScriptBlock = {
+    $global:GitPromptValues.DollarQuestion = $global:?
+    $global:GitPromptValues.LastExitCode = $origLastExitCode = $global:LASTEXITCODE
+    $global:GitPromptValues.IsAdmin = $IsAdmin
+
     $settings = $global:GitPromptSettings
     if (!$settings) {
         return "<`$GitPromptSettings not found> "
@@ -38,8 +42,6 @@ $GitPromptScriptBlock = {
         $sw = [System.Diagnostics.Stopwatch]::StartNew()
     }
 
-    $origLastExitCode = $global:LASTEXITCODE
-
     if ($settings.SetEnvColumns) {
         # Set COLUMNS so git knows how wide the terminal is
         $Env:COLUMNS = $Host.UI.RawUI.WindowSize.Width
@@ -100,8 +102,7 @@ $GitPromptScriptBlock = {
     }
     else {
         # If using ANSI, set this global to help debug ANSI issues
-        [System.Diagnostics.CodeAnalysis.SuppressMessageAttribute('PSUseDeclaredVarsMoreThanAssigments', '')]
-        $global:PoshGitLastPrompt = EscapeAnsiString $prompt
+        $global:GitPromptValues.LastPrompt = EscapeAnsiString $prompt
     }
 
     $global:LASTEXITCODE = $origLastExitCode