Proper check to see if zip dest needs to be wrapped in tellable

From hgweb, calling archival.zipit fails with the error message
"Illegal seek". This happens because sys.stdout.tell() throws an
exception:

Traceback (most recent call last):
  File "/usr/lib/python2.4/site-packages/mercurial/archival.py", line 99, in addfile
    self.z.writestr(i, data)
  File "/usr/lib/python2.4/zipfile.py", line 468, in writestr
    zinfo.header_offset = self.fp.tell()    # Start of header bytes

Checking whether hasattr(dest, 'tell') is insufficient, because
sys.stdout has a tell() method; you just can't call it.

This patch instead determines whether a fileobj is tellable by trying
to tell(), wrapping the fileobj if an exception is generated.

mercurial/archival.py

@@ -80,7 +80,12 b' class zipit:'
 
     def __init__(self, dest, prefix, compress=True):
         self.prefix = tidyprefix(dest, prefix, ('.zip',))
-        if not isinstance(dest, str) and not hasattr(dest, 'tell'):
+        if not isinstance(dest, str):
+            try:
+                dest.tell()
+            except AttributeError:
+                dest = tellable(dest)
+            except IOError:
             dest = tellable(dest)
         self.z = zipfile.ZipFile(dest, 'w',
                                  compress and zipfile.ZIP_DEFLATED or