git-gui: Switch the git-gui logo to Henrik Nyh's logo

git-gui: Switch the git-gui logo to Henrik Nyh's logo

Henrik came up with this alternative logo for gitweb and posted
it on his blog:

  http://henrik.nyh.se/2007/06/alternative-git-logo-and-favicon

The msysGit port uses his logo within some of their components,
and frankly it looks better here in git-gui for our repository
setup wizard screen.  The logo fits quite nicely along the left
edge of our window, leaving significantly more vertical space
for things like the git-fetch console output.

Because the logo changes the layout charateristics of the setup
window I also needed to adjust some of the padding for our widgets
and stop using a fixed width window size.  We now let Tk compute
the correct size of the main window whenever the layout changes,
and drop the window into roughly the upper left 1/3 of the desktop
so its not quite centered but is likely to be far enough away from
any sort of task bars/menu bars/docks that the user may have along
any edge of the screen.

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