fncache: make it possible to ignore some file

In the next changeset, we need to able to ignore some temporary file. This
changeset teach the fncache about that.

mercurial/store.py

@@ -603,6 +603,7 b' class fncache:'
     # hence the encodedir/decodedir dance
     def __init__(self, vfs):
         self.vfs = vfs
+        self._ignores = set()
         self.entries = None
         self._dirty = False
         # set of new additions to fncache
@@ -687,7 +688,12 b' class fncache:'
             self.entries = None
             self.addls = set()
 
+    def addignore(self, fn):
+        self._ignores.add(fn)
+
     def add(self, fn):
+        if fn in self._ignores:
+            return
         if self.entries is None:
             self._load()
         if fn not in self.entries: