store: only add new entries to the fncache file

Newly added fncache entries were not added to the in-memory cache,
making it possible for 'hg convert' to cause duplicates in
.hg/store/fncache.

Duplicates in the fncache file are harmless, but excessive numbers
of duplicates from large converted repositories may slow down
execution speed considerably.

mercurial/store.py

@@ -267,7 +267,9 b' class fncache(object):'
     def add(self, fn):
         if self.entries is None:
             self._load()
+        if fn not in self.entries:
         self.opener('fncache', 'ab').write(encodedir(fn) + '\n')
+            self.entries.add(fn)
 
     def __contains__(self, fn):
         if self.entries is None:
@@ -290,9 +292,7 b' class fncachestore(basicstore):'
         self.fncache = fnc
 
         def fncacheopener(path, mode='r', *args, **kw):
-            if (mode not in ('r', 'rb')
-                and path.startswith('data/')
-                and path not in fnc):
+            if mode not in ('r', 'rb') and path.startswith('data/'):
                 fnc.add(path)
             return op(hybridencode(path), mode, *args, **kw)
         self.opener = fncacheopener

tests/test-convert

@@ -56,4 +56,10 b' hg convert --source-type foo a a-foo'
 echo % convert with imaginary sink type
 hg convert --dest-type foo a a-foo
 
+echo
+echo % "testing: convert must not produce duplicate entries in fncache"
+hg convert a b
+echo % "contents of fncache file:"
+cat b/.hg/store/fncache
+
 true

tests/test-convert.out

@@ -273,3 +273,17 b' initializing destination a-foo repositor'
 abort: foo: invalid source repository type
 % convert with imaginary sink type
 abort: foo: invalid destination repository type
+
+% testing: convert must not produce duplicate entries in fncache
+initializing destination b repository
+scanning source...
+sorting...
+converting...
+4 a
+3 b
+2 c
+1 d
+0 e
+% contents of fncache file:
+data/a.i
+data/b.i