git-gui: Bind M1-N to create branch.

Creating branches is a common enough activity within a Git project
that we probably should give it a keyboard accelerator.  N is not
currently used and seems reasonable to stand for "New Branch". To
bad our menu calls it create.

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

git-gui.sh

@@ -3248,6 +3248,7 @@ if {!$single_commit} {
 
 	.mbar.branch add command -label {Create...} \
 		-command do_create_branch \
+		-accelerator $M1T-N \
 		-font font_ui
 	lappend disable_on_lock [list .mbar.branch entryconf \
 		[.mbar.branch index last] -state]
@@ -3815,6 +3816,11 @@ bind $ui_diff <Key-Down>   {catch {%W yview scroll  1 units};break}
 bind $ui_diff <Key-Left>   {catch {%W xview scroll -1 units};break}
 bind $ui_diff <Key-Right>  {catch {%W xview scroll  1 units};break}
 
+if {!$single_commit} {
+	bind . <$M1B-Key-n> do_create_branch
+	bind . <$M1B-Key-N> do_create_branch
+}
+
 bind .   <Destroy> do_quit
 bind all <Key-F5> do_rescan
 bind all <$M1B-Key-r> do_rescan