Fix Get-CurrentBranch to always rely on git (and not GITHUB_REF env v…

…ariable) (microsoft#950)

Fix `Get-CurrentBranch` to always rely on git (and not GITHUB_REF env
variable)

`$env:GITHUB_REF` is not the same as the checked out branch.
`$env:GITHUB_REF` is the branch the workflow is running on.

Fixes
[AB#525684](https://dynamicssmb2.visualstudio.com/1fcb79e7-ab07-432a-a3c6-6cf5a88ba4a5/_workitems/edit/525684)

build/scripts/EnlistmentHelperFunctions.psm1

@@ -20,9 +20,6 @@ function Get-BuildMode() {
 }
 
 function Get-CurrentBranch() {
-    if ($ENV:GITHUB_REF) {
-        return $ENV:GITHUB_REF.Replace("refs/heads/", "")
-    }
     return git rev-parse --abbrev-ref HEAD
 }
 
@@ -327,4 +324,4 @@ function RunAndCheck {
 }
 
 Export-ModuleMember -Function *-*
-Export-ModuleMember -Function RunAndCheck
+Export-ModuleMember -Function RunAndCheck