src/Pure/System/isabelle_system.scala

         cwd = base_dir).check_rc(_ <= 1)
       File.read(patch)
     }
   }
 
+  def git_clone(url: String, target: Path,
+    checkout: String = "HEAD",
+    ssh: SSH.System = SSH.Local,
+    progress: Progress = new Progress
+  ): Unit = {
+    progress.echo("Cloning " + quote(url))
+    bash(
+      "git clone --quiet --no-checkout " + Bash.string(url) + " . && " +
+      "git checkout --quiet --detach " + Bash.string(checkout),
+      ssh = ssh, cwd = ssh.make_directory(target)).check
+  }
+
   def open(arg: String): Unit =
     bash("exec \"$ISABELLE_OPEN\" " + Bash.string(arg) + " >/dev/null 2>/dev/null &")
 
   def pdf_viewer(arg: Path): Unit =
     bash("exec \"$PDF_VIEWER\" " + File.bash_path(arg) + " >/dev/null 2>/dev/null &")