changelog: use "vfs.fstat()" instead of "util.fstat()"

Just invoking "os.fstat()" with "file.fileno()" doesn't require non
ANSI file API, because filename is not used for invocation of
"os.fstat()".

But "util.fstat()" should invoke "os.stat()" with "fp.name", if file
object doesn't have "fileno()" method for portability, and "fp.name"
may cause invocation of non ANSI file API.

So, this patch makes the constructor of appender class invoke
"util.fstat()" via vfs, to encapsulate filename handling.

mercurial/changelog.py

@@ -59,11 +59,12 b' def stripdesc(desc):'
 class appender(object):
     '''the changelog index must be updated last on disk, so we use this class
     to delay writes to it'''
-    def __init__(self, fp, buf):
+    def __init__(self, vfs, name, mode, buf):
         self.data = buf
+        fp = vfs(name, mode)
         self.fp = fp
         self.offset = fp.tell()
-        self.size = util.fstat(fp).st_size
+        self.size = vfs.fstat(fp).st_size
 
     def end(self):
         return self.size + len("".join(self.data))
@@ -114,7 +115,7 b' def delayopener(opener, target, divert, '
         if divert:
             return opener(name + ".a", mode.replace('a', 'w'))
         # otherwise, divert to memory
-        return appender(opener(name, mode), buf)
+        return appender(opener, name, mode, buf)
     return o
 
 class changelog(revlog.revlog):

mercurial/scmutil.py

@@ -245,6 +245,9 b' class abstractvfs(object):'
     def exists(self, path=None):
         return os.path.exists(self.join(path))
 
+    def fstat(self, fp):
+        return util.fstat(fp)
+
     def isdir(self, path=None):
         return os.path.isdir(self.join(path))