git-gui: Shorten the staged/unstaged changes title bar text

git-gui: Shorten the staged/unstaged changes title bar text

The titles for the staged and unstaged areas were usually opening
up too narrow by default, causing the text to be clipped by Tcl as
it tried to center the text in the middle of the available area.
This meant that users who were new to git-gui did not get to see
the entire header and may be unclear about what the different lists
are.

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