git-gui: Clear undo/redo stack when loading a message file from disk.

If we load a message file (e.g. MERGE_MSG) or we have just finished
making a commit and are clearing out the commit buffer we should also
clear out the undo/redo stack associated with that buffer.  The prior
undo/redo stack has no associated with the new content and therefore
is not useful to the user.

Also modified the sign-off operation to perform the entire update in
a single undo/redo operation, allowing the user to undo the signoff
in case they didn't actually want to do that.

I also noticed what may be a crash on Windows related to the up and
down arrow keys navigating within the diff viewer.  Since I got back
no stack trace (just an application exit with a loss of the commit
message) I suspect that the binding to scroll the text widget crashed
with an error and the wish process just terminated.  So now we catch
(and ignore) any sort of error related to the arrow keys in the diff
viewer.

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

git-gui

@@ -179,6 +179,7 @@ proc update_status {{final Ready.}} {
 		} elseif {[load_message SQUASH_MSG]} {
 		}
 		$ui_comm edit modified false
+		$ui_comm edit reset
 	}
 
 	if {$cfg_trust_mtime == {true}} {
@@ -503,6 +504,7 @@ proc load_last_commit {} {
 		$ui_comm delete 0.0 end
 		$ui_comm insert end $msg
 		$ui_comm edit modified false
+		$ui_comm edit reset
 		update_status
 	} else {
 		error_popup {You can't amend a merge commit.}
@@ -698,6 +700,7 @@ proc commit_stage2 {fd_wt curHEAD msg} {
 
 	$ui_comm delete 0.0 end
 	$ui_comm edit modified false
+	$ui_comm edit reset
 
 	if {$single_commit} do_quit
 
@@ -1463,8 +1466,9 @@ proc do_signoff {} {
 
 	set str "Signed-off-by: $GIT_COMMITTER_IDENT"
 	if {[$ui_comm get {end -1c linestart} {end -1c}] != $str} {
-		$ui_comm insert end "\n"
-		$ui_comm insert end $str
+		$ui_comm edit separator
+		$ui_comm insert end "\n$str"
+		$ui_comm edit separator
 		$ui_comm see end
 	}
 }
@@ -1733,6 +1737,7 @@ trace add variable commit_type write {uplevel #0 {
 }}
 text $ui_comm -background white -borderwidth 1 \
 	-undo true \
+	-maxundo 20 \
 	-autoseparators true \
 	-relief sunken \
 	-width 75 -height 9 -wrap none \
@@ -1836,10 +1841,10 @@ bind $ui_diff <$M1B-Key-v> {break}
 bind $ui_diff <$M1B-Key-V> {break}
 bind $ui_diff <$M1B-Key-a> {%W tag add sel 0.0 end;break}
 bind $ui_diff <$M1B-Key-A> {%W tag add sel 0.0 end;break}
-bind $ui_diff <Key-Up>     {%W yview scroll -1 units}
-bind $ui_diff <Key-Down>   {%W yview scroll 1 units}
-bind $ui_diff <Key-Left>   {%W xview scroll -1 units}
-bind $ui_diff <Key-Right>  {%W xview scroll 1 units}
+bind $ui_diff <Key-Up>     {catch {%W yview scroll -1 units};break}
+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}
 
 bind .   <Destroy> do_quit
 bind all <Key-F5> do_rescan