diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
index 857b980a3..51b5a45ac 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java
@@ -101,6 +101,8 @@ public static FS detect(Boolean cygwinUsed) {
 
 	private final File userHome;
 
+	private volatile Holder<File> gitPrefix;
+
 	/**
 	 * Constructs a file system abstraction.
 	 */
@@ -258,7 +260,29 @@ protected static String readPipe(File dir, String[] command, String encoding) {
 	}
 
 	/** @return the $prefix directory C Git would use. */
-	public abstract File gitPrefix();
+	public File gitPrefix() {
+		Holder<File> p = gitPrefix;
+		if (p == null) {
+			p = new Holder<File>(discoverGitPrefix());
+			gitPrefix = p;
+		}
+		return p.value;
+	}
+
+	/** @return the $prefix directory C Git would use. */
+	protected abstract File discoverGitPrefix();
+
+	/**
+	 * 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;
+	}
 
 	/**
 	 * Initialize a ProcesssBuilder to run a command using the system shell.
@@ -273,4 +297,12 @@ protected static String readPipe(File dir, String[] command, String encoding) {
 	 *         populating directory, environment, and then start the process.
 	 */
 	public abstract ProcessBuilder runInShell(String cmd, String[] args);
+
+	private static class Holder<V> {
+		final V value;
+
+		Holder(V value) {
+			this.value = value;
+		}
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java
index 14fac15df..180b016be 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java
@@ -52,7 +52,7 @@
 
 abstract class FS_POSIX extends FS {
 	@Override
-	public File gitPrefix() {
+	protected File discoverGitPrefix() {
 		String path = SystemReader.getInstance().getenv("PATH");
 		File gitExe = searchPath(path, "git");
 		if (gitExe != null)
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java
index 7e1b6434c..5838fed6a 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java
@@ -64,9 +64,6 @@ public String run() {
 				&& StringUtils.toLowerCase(osDotName).indexOf("windows") != -1;
 	}
 
-	private File gitPrefix;
-	private boolean gitPrefixEvaluated;
-
 	public boolean supportsExecute() {
 		return false;
 	}
@@ -85,27 +82,21 @@ public boolean retryFailedLockFileCommit() {
 	}
 
 	@Override
-	public File gitPrefix() {
-		if (gitPrefixEvaluated)
-			return gitPrefix;
-
+	protected File discoverGitPrefix() {
 		String path = SystemReader.getInstance().getenv("PATH");
 		File gitExe = searchPath(path, "git.exe", "git.cmd");
 		if (gitExe != null)
-			gitPrefix = gitExe.getParentFile().getParentFile();
-		else {
-			// This isn't likely to work, if bash is in $PATH, git should
-			// also be in $PATH. But its worth trying.
-			//
-			String w = readPipe(userHome(), //
-			                    new String[] { "bash", "--login", "-c", "which git" }, //
-			                    Charset.defaultCharset().name());
-			if (w != null)
-				gitPrefix = new File(w).getParentFile().getParentFile();
-		}
+			return gitExe.getParentFile().getParentFile();
 
-		gitPrefixEvaluated = true;
-		return gitPrefix;
+		// This isn't likely to work, if bash is in $PATH, git should
+		// also be in $PATH. But its worth trying.
+		//
+		String w = readPipe(userHome(), //
+				new String[] { "bash", "--login", "-c", "which git" }, //
+				Charset.defaultCharset().name());
+		if (w != null)
+			return new File(w).getParentFile().getParentFile();
+		return null;
 	}
 
 	@Override