Get real source branch name in VSTS Pull Request

VSTS now has System.PullRequest.SourceBranch and System.PullRequest.TargetBranch variables. That should solve your problem without writing any custom scripts

Build Variables

This script will read the parameters from environment variables if used inside a build task or use the supplied parameters if used outside a build task. The variable $sourcebranch will be set to be used in a later build task.

[CmdletBinding()]
param (
    [Parameter(Mandatory=$false)][string]$projectUri = "$env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI",
    [Parameter(Mandatory=$false)][string]$branch = "$env:BUILD_branch",
    [Parameter(Mandatory=$false)][string]$repositoryName = "$env:BUILD_REPOSITORY_NAME",
    [Parameter(Mandatory=$false)][string]$projectName = "$env:SYSTEM_TEAMPROJECT",
    [Parameter(Mandatory=$false)][string]$oAuthToken = "$env:SYSTEM_ACCESSTOKEN",
    [Parameter(Mandatory=$false)][string]$username,
    [Parameter(Mandatory=$false)][string]$password
)

#check all parameters
if(!$oAuthToken) {
    if(!$username -or !$password) {
        throw "You must either supply an OAuth Token or a username and a password. You can supply the token via the environment variable SYSTEM_ACCESSTOKEN"
    }

    $basicAuth= ("{0}:{1}"-f $username,$password)
    $basicAuth=[System.Text.Encoding]::UTF8.GetBytes($basicAuth)
    $basicAuth=[System.Convert]::ToBase64String($basicAuth)
    $headers= @{Authorization=("Basic {0}"-f $basicAuth)}
}
else {
    $headers= @{Authorization="Bearer $oAuthToken"}
}

if(!$projectUri) {
    throw "You must supply a project uri or set the Environment variable SYSTEM_TEAMFOUNDATIONCOLLECTIONURI"
}

if(!$branch) {
    throw "You must supply a branch or set the Environment variable BUILD_branch"
}

if(!$repositoryName) {
    throw "You must supply a repository name or set the Environment variable BUILD_REPOSITORY_NAME"
}

if(!$projectName) {
    throw "You must supply a project name or set the Environment variable SYSTEM_TEAMPROJECT"
}

#get pull request ID via regex
$pullrequest = "refs/pull/+(?<pullnumber>\w+?)/merge+"
if($branch -match $pullrequest) {        
    $pullrequestid = $Matches.pullnumber;
    Write-Output "Pull request ID is $pullrequestid"
}
else { 
    Write-Output "Cannot find pull request ID" 
}

#get pull request information via API
$url= $projectUri + "DefaultCollection/$projectName/_apis/git/repositories/$repositoryName/pullRequests/$pullrequestid\?api-version=1.0-preview.1"
Write-Output "Getting info from $url"
$getpullrequest = Invoke-RestMethod -Uri $url -headers $headers -Method Get

#get sourcebranch and targetbranch ref
$sourceref = $getpullrequest.sourceRefName
$targetref = $getpullrequest.targetRefName

#get the branch name via regex
$branchref = "refs/heads/(?<realBranchname>.*)"
if($sourceref -match $branchref) {        
    $sourcebranch = $Matches.realBranchname;
    Write-Output "Real source branch is $sourcebranch"
}
else { 
    Write-Output "Cannot find real source branch" 
}
if($targetref -match $branchref) {        
    $targetbranch = $Matches.realBranchname;
    Write-Output "Real target branch is $targetbranch"
}
else { 
    Write-Output "Cannot find real target branch" 
}

#set a variable "sourcebranch" to use it in another build task
Write-Output "##vso[task.setvariable variable=sourcebranch;]$sourcebranch"

You can create a bash script that assigns the shorter branch name to a variable.

# Bash script
BRANCH_NAME=$(echo "$(System.PullRequest.TargetBranch)" | awk -F/ '{print $NF}')
echo "##vso[task.setvariable variable=PullRequest_Target_Branch;]$BRANCH_NAME"

You can then reference $(PullRequest_Target_Branch) in your pipeline

There isn't any variable for this but you can create a power-shell script to get it via Rest API.

[String]$projecturi = "$env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI"
[String]$sourcebranch = "$env:BUILD_SOURCEBRANCH"
[String]$repoid = "$env:BUILD_REPOSITORY_ID"

$username="alternativeusername"
$password="alternativepassword"

$basicAuth= ("{0}:{1}"-f $username,$password)
$basicAuth=[System.Text.Encoding]::UTF8.GetBytes($basicAuth)
$basicAuth=[System.Convert]::ToBase64String($basicAuth)
$headers= @{Authorization=("Basic {0}"-f $basicAuth)}

#get pull request ID via regex
$pullrequest = "refs/pull/+(?<pullnumber>\w+?)/merge+"
if($sourcebranch -match $pullrequest){        
        $pullrequestid = $Matches.pullnumber;
    }
else { write-host "Cannot find pull request ID" }

#get pull request information via API
$url= $projecturi + "_apis/git/repositories/" + $repoid + "/pullRequests/" + $pullrequestid + "?api-version=1.0-preview.1"

Write-Host $url

$getpullrequest = Invoke-RestMethod -Uri $url -headers $headers -Method Get

#get sourcebranch and targetbranch
$sourceref = $getpullrequest.sourceRefName
$targetref = $getpullrequest.targetRefName