git-gui: convert new/amend commit radiobutton to checkbutton

Its a bi-state anyway and also saves one line in the menu.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>
diff --git a/lib/commit.tcl b/lib/commit.tcl
index 83620b7..384f18f 100644
--- a/lib/commit.tcl
+++ b/lib/commit.tcl
@@ -327,7 +327,7 @@
 proc commit_committree {fd_wt curHEAD msg_p} {
 	global HEAD PARENT MERGE_HEAD commit_type commit_author
 	global current_branch
-	global ui_comm selected_commit_type
+	global ui_comm commit_type_is_amend
 	global file_states selected_paths rescan_active
 	global repo_config
 	global env
@@ -461,8 +461,8 @@
 
 	# -- Update in memory status
 	#
-	set selected_commit_type new
 	set commit_type normal
+	set commit_type_is_amend 0
 	set HEAD $cmt_id
 	set PARENT $cmt_id
 	set MERGE_HEAD [list]