FS: Allow to manually set the path to the Git system config file

Now that d7a4473 removed the gitprefix property, we did not have a way to
specify the path to the Git system config file in case
discoverGitSystemConfig() fails. Fix that by introducing a member variable
that caches the result of discoverGitSystemConfig() as well as a setter
method to overwrite the content of that variable.

Change-Id: Icd965bffbe2f11b18c9505ee2ddd2afad5b64d70
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -157,6 +157,8 @@ public abstract class FS {
 
 	private volatile Holder<File> userHome;
 
+	private volatile Holder<File> gitSystemConfig;
+
 	/**
 	 * Constructs a file system abstraction.
 	 */
@@ -172,6 +174,7 @@ public abstract class FS {
 	 */
 	protected FS(FS src) {
 		userHome = src.userHome;
+		gitSystemConfig = src.gitSystemConfig;
 	}
 
 	/** @return a new instance of the same type of FS. */
@@ -547,6 +550,31 @@ public abstract class FS {
 		return new File(w);
 	}
 
+	/**
+	 * @return the currently used path to the system-wide Git configuration
+	 *         file or {@code null} if none has been set.
+	 * @since 4.0
+	 */
+	public File getGitSystemConfig() {
+		if (gitSystemConfig == null) {
+			gitSystemConfig = new Holder<File>(discoverGitSystemConfig());
+		}
+		return gitSystemConfig.value;
+	}
+
+	/**
+	 * Set the path to the system-wide Git configuration file to use.
+	 *
+	 * @param configFile
+	 *            the path to the config file.
+	 * @return {@code this}
+	 * @since 4.0
+	 */
+	public FS setGitSystemConfig(File configFile) {
+		gitSystemConfig = new Holder<File>(configFile);
+		return this;
+	}
+
 	/**
 	 * @param grandchild
 	 * @return the parent directory of this file's parent directory or

org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java

@@ -89,7 +89,7 @@ public abstract class SystemReader {
 		}
 
 		public FileBasedConfig openSystemConfig(Config parent, FS fs) {
-			File configFile = fs.discoverGitSystemConfig();
+			File configFile = fs.getGitSystemConfig();
 			if (configFile == null) {
 				return new FileBasedConfig(null, fs) {
 					public void load() {