gitk: make the "list references" default window width wider

gitk: make the "list references" default window width wider

When using remotes (with git-flow especially), the remote reference names
are almost always wordwrapped in the "list references" window because it's
somewhat narrow by default. It's possible to resize it with a mouse,
but it's annoying to have to do this every time, especially on Windows 10,
where the window border seems to be only one (1) pixel wide, thus making
the grabbing of the window border tricky.

Signed-off-by: James J. Raden <james.raden@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>