git-gui--askyesno: allow overriding the window title

git-gui--askyesno: allow overriding the window title

"Question?" is maybe not the most informative thing to ask. In the
absence of better information, it is the best we can do, of course.

However, Git for Windows' auto updater just learned the trick to use
git-gui--askyesno to ask the user whether to update now or not. And in
this scripted scenario, we can easily pass a command-line option to
change the window title.

So let's support that with the new `--title <title>` option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>