git-gui: Option to default new branches to match tracking branches

git-gui: Option to default new branches to match tracking branches

In some workflows users will want to almost always just create a new
local branch that matches a remote branch.  In this type of workflow
it is handy to have the new branch dialog default to "Match Tracking
Branch" and "Starting Revision"-Tracking Branch", with the focus in
the branch filter field.  This can save users working on this type
of workflow at least two mouse clicks every time they create a new
local branch or switch to one with a fast-forward.

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