Fix detection of the root package version to always use the correct cwd

src/Composer/Factory.php

@@ -275,7 +275,7 @@ class Factory
         $parser = new VersionParser;
         $guesser = new VersionGuesser($config, new ProcessExecutor($io), $parser);
         $loader  = new Package\Loader\RootPackageLoader($rm, $config, $parser, $guesser);
-        $package = $loader->load($localConfig);
+        $package = $loader->load($localConfig, 'Composer\Package\RootPackage', $cwd);
         $composer->setPackage($package);
 
         // initialize installation manager

src/Composer/Package/Loader/RootPackageLoader.php

@@ -13,6 +13,7 @@
 namespace Composer\Package\Loader;
 
 use Composer\Package\BasePackage;
+use Composer\Package\PackageInterface;
 use Composer\Package\AliasPackage;
 use Composer\Config;
 use Composer\Factory;
@@ -54,7 +55,13 @@ class RootPackageLoader extends ArrayLoader
         $this->versionGuesser = $versionGuesser ?: new VersionGuesser($config, new ProcessExecutor(), $this->versionParser);
     }
 
-    public function load(array $config, $class = 'Composer\Package\RootPackage')
+    /**
+     * @param array $config package data
+     * @param string $class FQCN to be instantiated
+     * @param string $cwd cwd of the root package to be used to guess the version if it is not provided
+     * @return PackageInterface
+     */
+    public function load(array $config, $class = 'Composer\Package\RootPackage', $cwd = null)
     {
         if (!isset($config['name'])) {
             $config['name'] = '__root__';
@@ -65,7 +72,7 @@ class RootPackageLoader extends ArrayLoader
             if (getenv('COMPOSER_ROOT_VERSION')) {
                 $version = getenv('COMPOSER_ROOT_VERSION');
             } else {
-                $version = $this->versionGuesser->guessVersion($config, getcwd());
+                $version = $this->versionGuesser->guessVersion($config, $cwd ?: getcwd());
             }
 
             if (!$version) {