Move resolveGrandparentFile() to the base class for wider use

Change-Id: I67ec732ea2e5345a6946783f0c5ef60c07ce254e
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -507,6 +507,21 @@ public abstract class FS {
 	/** @return the $prefix directory C Git would use. */
 	protected abstract File discoverGitPrefix();
 
+	/**
+	 * @param grandchild
+	 * @return the parent directory of this file's parent directory or
+	 *         {@code null} in case there's no grandparent directory
+	 * @since 4.0
+	 */
+	protected static File resolveGrandparentFile(File grandchild) {
+		if (grandchild != null) {
+			File parent = grandchild.getParentFile();
+			if (parent != null)
+				return parent.getParentFile();
+		}
+		return null;
+	}
+
 	/**
 	 * Set the $prefix directory C Git uses.
 	 *

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

@@ -138,7 +138,7 @@ public class FS_POSIX extends FS {
 		String path = SystemReader.getInstance().getenv("PATH"); //$NON-NLS-1$
 		File gitExe = searchPath(path, "git"); //$NON-NLS-1$
 		if (gitExe != null)
-			return gitExe.getParentFile().getParentFile();
+			return resolveGrandparentFile(gitExe);
 
 		if (SystemReader.getInstance().isMacOS()) {
 			// On MacOSX, PATH is shorter when Eclipse is launched from the
@@ -150,10 +150,7 @@ public class FS_POSIX extends FS {
 					Charset.defaultCharset().name());
 			if (w == null || w.length() == 0)
 				return null;
-			File parentFile = new File(w).getParentFile();
-			if (parentFile == null)
-				return null;
-			return parentFile.getParentFile();
+			return resolveGrandparentFile(new File(w));
 		}
 
 		return null;

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java

@@ -128,15 +128,6 @@ public class FS_Win32 extends FS {
 		return null;
 	}
 
-	private static File resolveGrandparentFile(File grandchild) {
-		if (grandchild != null) {
-			File parent = grandchild.getParentFile();
-			if (parent != null)
-				return parent.getParentFile();
-		}
-		return null;
-	}
-
 	@Override
 	protected File userHomeImpl() {
 		String home = SystemReader.getInstance().getenv("HOME"); //$NON-NLS-1$