git-gui: Increase the default height of the revision picker

git-gui: Increase the default height of the revision picker

Showing only five lines of heads/tags is not very useful to a user
when they have about 10 branches that match the filter expression.
The list is just too short to really be able to read easily, at
least not without scrolling up and down.  Expanding the list out
to 10 really makes the revision picker easier to read and access,
as you can read the matching branches much more quickly.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>