Clean up git env vars consistently

src/Composer/Downloader/GitDownloader.php

@@ -77,6 +77,7 @@ class GitDownloader extends VcsDownloader
      */
     public function getLocalChanges($path)
     {
+        $this->cleanEnv();
         $path = $this->normalizePath($path);
         if (!is_dir($path.'/.git')) {
             return;
@@ -95,6 +96,7 @@ class GitDownloader extends VcsDownloader
      */
     protected function cleanChanges($path, $update)
     {
+        $this->cleanEnv();
         $path = $this->normalizePath($path);
         if (!$changes = $this->getLocalChanges($path)) {
             return;

src/Composer/Util/Git.php

@@ -26,10 +26,16 @@ class Git
         }
 
         // added in git 1.7.1, prevents prompting the user for username/password
-        putenv('GIT_ASKPASS=echo');
+        if (getenv('GIT_ASKPASS') !== 'echo') {
+            putenv('GIT_ASKPASS=echo');
+        }
 
         // clean up rogue git env vars in case this is running in a git hook
-        putenv('GIT_DIR');
-        putenv('GIT_WORK_TREE');
+        if (getenv('GIT_DIR')) {
+            putenv('GIT_DIR');
+        }
+        if (getenv('GIT_WORK_TREE')) {
+            putenv('GIT_WORK_TREE');
+        }
     }
 }