last_engine_commit.ps1: Use $flutterRoot instead of $gitTopLevel (flutter#172786)

Closes flutter#172190.

Author: matanlurey

bin/internal/last_engine_commit.ps1

@@ -24,9 +24,8 @@ $ErrorActionPreference = "Stop" # Equivalent to 'set -e' in bash
 
 $progName = Split-Path -parent $MyInvocation.MyCommand.Definition
 $flutterRoot = (Get-Item $progName).parent.parent.FullName
-$gitToplevel = (git rev-parse --show-toplevel).Trim()
 
-$Path1 = Join-Path $gitToplevel "bin"
+$Path1 = Join-Path $flutterRoot "bin"
 $Path2 = Join-Path $Path1 "internal"
 $RELEASE_CANDIDATE_VERSION_PATH = Join-Path $Path2 "release-candidate-branch.version"
 
@@ -53,8 +52,8 @@ if ([string]::IsNullOrEmpty($REFERENCE_COMMIT)) {
 #    but not from the REFERENCE_COMMIT. This focuses the search on commits
 #    *unique to the current branch* since that file was last changed.
 $HISTORY_RANGE = "$REFERENCE_COMMIT..HEAD"
-$DEPS_PATH = Join-Path $gitToplevel "DEPS"
-$ENGINE_PATH = Join-Path $gitToplevel "engine"
+$DEPS_PATH = Join-Path $flutterRoot "DEPS"
+$ENGINE_PATH = Join-Path $flutterRoot "engine"
 
 $ENGINE_COMMIT = (git log -1 --pretty=format:%H --ancestry-path $HISTORY_RANGE -- "$DEPS_PATH" "$ENGINE_PATH")