fix appendfile problem on macos.

when i open file with mode 'a+' on linux, seek position is 0.
on macos seek position is end of file instead.  not documented.

mercurial/appendfile.py

@@ -37,7 +37,7 b' class appendfile(object):'
         fd, self.tmpname = tempfile.mkstemp()
         self.tmpfp = os.fdopen(fd, 'ab+')
         self.realfp = fp
-        self.offset = 0
+        self.offset = fp.tell()
         # real file is not written by anyone else. cache its size so
         # seek and read can be fast.
         self.fpsize = os.fstat(fp.fileno()).st_size