bundle-spec: add an official method to format a spec into a string

This will be useful for automatic-clonebundles.

mercurial/bundlecaches.py

@@ -89,6 +89,12 b' class bundlespec:'
         if overwrite or key not in self._explicit_params:
             self._explicit_params[key] = value
 
+    def as_spec(self):
+        parts = [b"%s-%s" % (self.compression, self.version)]
+        for param in sorted(self._explicit_params.items()):
+            parts.append(b'%s=%s' % param)
+        return b';'.join(parts)
+
 
 # Maps bundle version human names to changegroup versions.
 _bundlespeccgversions = {