Previously, the focused window in each tab was the Fugitive window that
contained the old version of the file. It's nicer to have the new
version focused by default, because that allows you to make changes to
the local source code.

The Fugitive documentation says that `Gdiff` is deprecated. In the
current version, you need to use the `Gvdiffsplit` command to get a
side-by-side diff with a vertical split.

When the files being opened with `difftool` are already open in a Vim
instance, a 'swap file already exists' confirmation warning is displayed
for each file. Bypass the warning by not saving swap files (using the -n
command line option).

Turns out I don't need to worry about setting or unsetting `extglob` at
all because the script gets executed in a subshell. Obviously. Didn't
realise or test thoroughly at first.

If `extglob` was set, we'd always unset it. Check to see if it's set
before doing anything with it, and then restore it to its original
value.

This was a note preceding implementation, and is no longer needed.

Add the range filter to try to remove commits that we don't care about.
Still not sure if this will work in all cases, as sometimes `..` seemed
to give me the right collection of files and sometimes `...` did.

code-review-difftool:
Pass the merge base without range dots (`..` or `...`) to Vim Fugitive,
as it doesn't accept revisions with ranges. Not sure what to do if I get
a range with two revision names yet, though.

Turns on Bash's `extglob` in order to use `?(...)` to include both
patterns in the expansion.