Suppport the GIT_CONFIG_NOSYSTEM environment variable

Change-Id: If3cc05931683d396b5ae2ea8952adceeb9a82ab9

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Config.java

 import org.eclipse.jgit.lib.StoredConfig;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.eclipse.jgit.util.FS;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.SystemReader;
 import org.kohsuke.args4j.Option;
 
 			list(new FileBasedConfig(configFile, fs));
 			return;
 		}
-		if (system || isListAll())
+		if (system
+				|| (isListAll() && StringUtils.isEmptyOrNull(SystemReader
+						.getInstance()
+						.getenv(Constants.GIT_CONFIG_NOSYSTEM_KEY))))
 			list(SystemReader.getInstance().openSystemConfig(null, fs));
 		if (global || isListAll())
 			list(SystemReader.getInstance().openUserConfig(null, fs));

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java

 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.eclipse.jgit.storage.file.FileRepositoryBuilder;
+import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FileUtils;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.SystemReader;
 
 /**
 	public FileRepository(final BaseRepositoryBuilder options) throws IOException {
 		super(options);
 
-		systemConfig = SystemReader.getInstance().openSystemConfig(null, getFS());
+		if (StringUtils.isEmptyOrNull(SystemReader.getInstance().getenv(
+				Constants.GIT_CONFIG_NOSYSTEM_KEY)))
+			systemConfig = SystemReader.getInstance().openSystemConfig(null,
+					getFS());
+		else
+			systemConfig = new FileBasedConfig(null, FS.DETECTED) {
+				public void load() {
+					// empty, do not load
+				}
+
+				public boolean isOutdated() {
+					// regular class would bomb here
+					return false;
+				}
+			};
 		userConfig = SystemReader.getInstance().openUserConfig(systemConfig,
 				getFS());
 		repoConfig = new FileBasedConfig(userConfig, getFS().resolve(

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

 	/** The environment variable that contains the commiter's email */
 	public static final String GIT_COMMITTER_EMAIL_KEY = "GIT_COMMITTER_EMAIL";
 
+	/** The environment variable that blocks use of the system config file */
+	public static final String GIT_CONFIG_NOSYSTEM_KEY = "GIT_CONFIG_NOSYSTEM";
+
 	/**
 	 * The environment variable that limits how close to the root of the file
 	 * systems JGit will traverse when looking for a repository root.