Cache gitPrefix in FS_Win32

readPipe() may consume rather much time, so
gitPrefix should be cached. If the git executable changes,
users should run FS.detect() again to get a new
instance of FS_Win32.

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

@@ -64,6 +64,9 @@ class FS_Win32 extends FS {
 				&& StringUtils.toLowerCase(osDotName).indexOf("windows") != -1;
 	}
 
+	private File gitPrefix;
+	private boolean gitPrefixEvaluated;
+
 	public boolean supportsExecute() {
 		return false;
 	}
@@ -83,11 +86,14 @@ class FS_Win32 extends FS {
 
 	@Override
 	public File gitPrefix() {
+		if (gitPrefixEvaluated)
+			return gitPrefix;
+
 		String path = SystemReader.getInstance().getenv("PATH");
 		File gitExe = searchPath(path, "git.exe", "git.cmd");
 		if (gitExe != null)
-			return gitExe.getParentFile().getParentFile();
-
+			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.
 			//
@@ -95,9 +101,11 @@ class FS_Win32 extends FS {
 			                    new String[] { "bash", "--login", "-c", "which git" }, //
 			                    Charset.defaultCharset().name());
 			if (w != null)
-			return new File(w).getParentFile().getParentFile();
+				gitPrefix = new File(w).getParentFile().getParentFile();
+		}
 
-		return null;
+		gitPrefixEvaluated = true;
+		return gitPrefix;
 	}
 
 	@Override