Make 'Start-PSBuild' and 'Start-PSPackage' accept a release tag argument (#3921)

build.psm1

@@ -128,7 +128,12 @@ function Start-PSBuild {
         [switch]$Publish,
 
         [Parameter(ParameterSetName='CoreCLR')]
-        [switch]$CrossGen
+        [switch]$CrossGen,
+
+        [Parameter(ParameterSetName='CoreCLR')]
+        [ValidatePattern("^v\d+\.\d+\.\d+(-\w+\.\d+)?$")]
+        [ValidateNotNullOrEmpty()]
+        [string]$ReleaseTag
     )
 
     function Stop-DevPowerShell {
@@ -157,8 +162,13 @@ function Start-PSBuild {
         }
     }
 
-    # save Git description to file for PowerShell to include in PSVersionTable
-    git --git-dir="$PSScriptRoot/.git" describe --dirty --abbrev=60 > "$psscriptroot/powershell.version"
+    # save git commit id to file for PowerShell to include in PSVersionTable
+    $gitCommitId = $ReleaseTag
+    if (-not $gitCommitId) {
+        # if ReleaseTag is not specified, use 'git describe' to get the commit id
+        $gitCommitId = git --git-dir="$PSScriptRoot/.git" describe --dirty --abbrev=60
+    }
+    $gitCommitId > "$psscriptroot/powershell.version"
 
     # create the telemetry flag file
     $null = new-item -force -type file "$psscriptroot/DELETE_ME_TO_DISABLE_CONSOLEHOST_TELEMETRY"
@@ -1206,8 +1216,14 @@ function Start-PSBootstrap {
 function Start-PSPackage {
     [CmdletBinding()]param(
         # PowerShell packages use Semantic Versioning http://semver.org/
+        [Parameter(ParameterSetName = "Version")]
         [string]$Version,
 
+        [Parameter(ParameterSetName = "ReleaseTag")]
+        [ValidatePattern("^v\d+\.\d+\.\d+(-\w+\.\d+)?$")]
+        [ValidateNotNullOrEmpty()]
+        [string]$ReleaseTag,
+
         # Package name
         [ValidatePattern("^powershell")]
         [string]$Name = "powershell",
@@ -1253,6 +1269,11 @@ function Start-PSPackage {
         throw "Please ensure you have run 'Start-PSBuild -Clean -CrossGen -Runtime $Runtime -Configuration $Configuration'!"
     }
 
+    # If ReleaseTag is specified, use the given tag to calculate Vesrion
+    if ($PSCmdlet.ParameterSetName -eq "ReleaseTag") {
+        $Version = $ReleaseTag -Replace '^v'
+    }
+
     # Use Git tag if not given a version
     if (-not $Version) {
         $Version = (git --git-dir="$PSScriptRoot/.git" describe) -Replace '^v'