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>
diff --git a/lib/branch_create.tcl b/lib/branch_create.tcl
index 375f575..df3f435 100644
--- a/lib/branch_create.tcl
+++ b/lib/branch_create.tcl
@@ -93,13 +93,14 @@
 	pack $w.options.checkout -anchor nw
 	pack $w.options -anchor nw -fill x -pady 5 -padx 5
 
-	set name $repo_config(gui.newbranchtemplate)
+	trace add variable @name_type write [cb _select]
 
-	bind $w <Visibility> "
-		grab $w
-		$w_name icursor end
-		focus $w_name
-	"
+	set name $repo_config(gui.newbranchtemplate)
+	if {[is_config_true gui.matchtrackingbranch]} {
+		set name_type match
+	}
+
+	bind $w <Visibility> [cb _visible]
 	bind $w <Key-Escape> [list destroy $w]
 	bind $w <Key-Return> [cb _create]\;break
 	tkwait window $w
@@ -361,4 +362,18 @@
 	return 1
 }
 
+method _select {args} {
+	if {$name_type eq {match}} {
+		$w_rev pick_tracking_branch
+	}
+}
+
+method _visible {} {
+	grab $w
+	if {$name_type eq {user}} {
+		$w_name icursor end
+		focus $w_name
+	}
+}
+
 }