Add XML output to `nix-store'.

* src/nix-store/Makefile.am (nix_store_SOURCES): Add `xmlgraph.cc' and
  `xmlgraph.hh'.

* src/nix-store/help.txt (Operations): Document `--xml'.

* src/nix-store/nix-store.cc (opQuery): Handle `--xml'.

* src/nix-store/xmlgraph.cc, src/nix-store/xmlgraph.hh: New files.

src/nix-store/xmlgraph.cc

@@ -0,0 +1,71 @@
+#include "xmlgraph.hh"
+#include "util.hh"
+#include "store-api.hh"
+
+#include <iostream>
+
+
+using std::cout;
+
+namespace nix {
+
+
+static inline const string & xmlQuote(const string & s)
+{
+    // Luckily, store paths shouldn't contain any character that needs to be
+    // quoted.
+    return s;
+}
+
+
+static string makeEdge(const string & src, const string & dst)
+{
+    format f = format("  <edge src=\"%1%\" dst=\"%2%\"/>\n")
+      % xmlQuote(src) % xmlQuote(dst);
+    return f.str();
+}
+
+
+static string makeNode(const string & id)
+{
+    format f = format("  <node name=\"%1%\"/>\n") % xmlQuote(id);
+    return f.str();
+}
+
+
+void printXmlGraph(const PathSet & roots)
+{
+    PathSet workList(roots);
+    PathSet doneSet;
+
+    cout << "<?xml version='1.0' encoding='utf-8'?>\n"
+	 << "<nix>\n";
+
+    while (!workList.empty()) {
+	Path path = *(workList.begin());
+	workList.erase(path);
+
+	if (doneSet.find(path) != doneSet.end()) continue;
+	doneSet.insert(path);
+
+	cout << makeNode(path);
+
+	PathSet references;
+	store->queryReferences(path, references);
+
+	for (PathSet::iterator i = references.begin();
+	     i != references.end(); ++i)
+	{
+	    if (*i != path) {
+		workList.insert(*i);
+		cout << makeEdge(*i, path);
+	    }
+	}
+
+    }
+
+    cout << "</nix>\n";
+}
+
+
+}