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>

git-gui.sh

@@ -1339,6 +1339,7 @@ set default_config(merge.verbosity) 2
 set default_config(user.name) {}
 set default_config(user.email) {}
 
+set default_config(gui.matchtrackingbranch) false
 set default_config(gui.pruneduringfetch) false
 set default_config(gui.trustmtime) false
 set default_config(gui.diffcontext) 5