FS: Remove the gitprefix logic

The only purpose of the gitprefix logic was to determine the path to the
system-wide config file. This is now done by discoverGitSystemConfig()
independent of the gitprefix, so get rid of this unused code.

Change-Id: Iaa88df9bd066dc1ed4067c18618af809e49876b3
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>

README.md

@@ -94,9 +94,9 @@ Warnings/Caveats
   Git is installed. Make sure Git can be found via the PATH
   environment variable. When installing Git for Windows check the "Run
   Git from the Windows Command Prompt" option. There are other options
-  like the jgit.gitprefix system property or Eclipse settings that can
+  like Eclipse settings that can be used for pointing out where C Git
-  be used for pointing out where C Git is installed. Modifying PATH is
+  is installed. Modifying PATH is the recommended option if C Git is
-  the recommended option if C Git is installed.
+  installed.
 
 - We try to use the same notation of $HOME as C Git does. On Windows
   this is often not the same value as the user.home system property.

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

@@ -157,8 +157,6 @@ public abstract class FS {
 
 	private volatile Holder<File> userHome;
 
-	private volatile Holder<File> gitPrefix;
-
 	/**
 	 * Constructs a file system abstraction.
 	 */
@@ -174,7 +172,6 @@ public abstract class FS {
 	 */
 	protected FS(FS src) {
 		userHome = src.userHome;
-		gitPrefix = src.gitPrefix;
 	}
 
 	/** @return a new instance of the same type of FS. */
@@ -517,21 +514,6 @@ public abstract class FS {
 		return null;
 	}
 
-	/** @return the $prefix directory C Git would use. */
-	public File gitPrefix() {
-		Holder<File> p = gitPrefix;
-		if (p == null) {
-			String overrideGitPrefix = SystemReader.getInstance().getProperty(
-					"jgit.gitprefix"); //$NON-NLS-1$
-			if (overrideGitPrefix != null)
-				p = new Holder<File>(new File(overrideGitPrefix));
-			else
-				p = new Holder<File>(discoverGitPrefix());
-			gitPrefix = p;
-		}
-		return p.value;
-	}
-
 	/**
 	 * @return the path to the Git executable.
 	 * @since 4.0
@@ -563,11 +545,6 @@ public abstract class FS {
 		return new File(w);
 	}
 
-	/** @return the $prefix directory C Git would use. */
-	protected File discoverGitPrefix() {
-		return resolveGrandparentFile(discoverGitExe());
-	}
-
 	/**
 	 * @param grandchild
 	 * @return the parent directory of this file's parent directory or
@@ -583,18 +560,6 @@ public abstract class FS {
 		return null;
 	}
 
-	/**
-	 * Set the $prefix directory C Git uses.
-	 *
-	 * @param path
-	 *            the directory. Null if C Git is not installed.
-	 * @return {@code this}
-	 */
-	public FS setGitPrefix(File path) {
-		gitPrefix = new Holder<File>(path);
-		return this;
-	}
-
 	/**
 	 * Check if a file is a symbolic link and read it
 	 *