treemanifest: extract parse method from constructor

When we start to lazily load submanifests, it will be useful to be
able to create an treemanifest instance before manifest data gets
parsed into it. To prepare for this, extract the parsing code from
treemanifest's constructor to a separate method.

mercurial/manifest.py

@@ -448,10 +448,7 b' class treemanifest(object):'
         # Using _lazymanifest here is a little slower than plain old dicts
         self._files = {}
         self._flags = {}
-        for f, n, fl in _parse(text):
-            self[f] = n
-            if fl:
-                self.setflag(f, fl)
+        self.parse(text)
 
     def _subpath(self, path):
         return self._dir + path
@@ -740,6 +737,12 b' class treemanifest(object):'
         _diff(self, m2)
         return result
 
+    def parse(self, text):
+        for f, n, fl in _parse(text):
+            self[f] = n
+            if fl:
+                self.setflag(f, fl)
+
     def text(self, usemanifestv2=False):
         """Get the full data of this manifest as a bytestring."""
         flags = self.flags