dirstate: introduce an internal `_add` method

We want to split current user of `dirstate.add` between `hg add`-like cases and
update of the dirstate coming from update/merge.

To do this we will introduce new API. The first step is to introduces an
internal function that these new API migh use (or not use) to distinct between
the migrated users and the others.

Differential Revision: https://phab.mercurial-scm.org/D11010

mercurial/dirstate.py

@@ -387,7 +387,7 b' class dirstate(object):'
                     source = self._map.copymap.get(f)
                     if source:
                         copies[f] = source
-                    self.add(f)
+                    self._add(f)
         return copies
 
     def setbranch(self, branch):
@@ -547,8 +547,12 b' class dirstate(object):'
 
     def add(self, f):
         '''Mark a file added.'''
-        self._addpath(f, added=True)
-        self._map.copymap.pop(f, None)
+        self._add(f)
+
+    def _add(self, filename):
+        """internal function to mark a file as added"""
+        self._addpath(filename, added=True)
+        self._map.copymap.pop(filename, None)
 
     def remove(self, f):
         '''Mark a file removed.'''