VimDiff => ":diffupdate" does not work

When changing one or both files which are currently open/compared in
VimDiff, in an *external* editor (so, not in Vim or VimDiff itself),
VimDiff doesn't update the diffs when forcing an update using the
command ":diffupdate". In fact, nothing happens at all while
executing this ":diffupdate" command.

Only when forcing VimDiff to lose focus and re-gain focus again, I see
the message that files have been changed. After (re)loading the
changed files, the diff is updated.

I enforce losing focus by executing the command ":shell" from within
VimDiff and then exit-ing the shell mode again to come back to
VimDiff.

Is ":diffupdate" not supposed to "forcefully" rescan both files and
update the view?

I'm using VimDiff version 7.1.138 on Ubuntu 8.04, but the same
complaint is valid for VimDiff 7.3.449 on a very recent Ubuntu 12.04
distro.

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