git-gui: Cleanup diff construction code to prepare for more options.

I'd like to allow the user to have more control over how we format
the diff in the diff viewer; to that end we need to add additional
options to the diff-index command line as we construct the command
for execution.

So cleanup the command handling code now to use lappend so we can
come back and add in our additional options.

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

git-gui

@@ -488,7 +488,7 @@ prevent possible confusion.
 
 proc show_diff {path {w {}} {lno {}}} {
 	global file_states file_lists
-	global PARENT diff_3way diff_active
+	global PARENT diff_3way diff_active repo_config
 	global ui_diff ui_fname_value ui_fstatus_value ui_status_value
 
 	if {$diff_active || ![lock_index read]} return
@@ -515,10 +515,13 @@ proc show_diff {path {w {}} {lno {}}} {
 	set ui_fstatus_value [mapdesc $m $path]
 	set ui_status_value "Loading diff of [escape_path $path]..."
 
-	set cmd [list | git diff-index -p $PARENT -- $path]
+	set cmd [list | git diff-index]
+	lappend cmd --no-color
+	lappend cmd -p
+
 	switch $m {
 	MM {
-		set cmd [list | git diff-index -p -c $PARENT $path]
+		lappend cmd -c
 	}
 	_O {
 		if {[catch {
@@ -542,6 +545,10 @@ proc show_diff {path {w {}} {lno {}}} {
 	}
 	}
 
+	lappend cmd $PARENT
+	lappend cmd --
+	lappend cmd $path
+
 	if {[catch {set fd [open $cmd r]} err]} {
 		set diff_active 0
 		unlock_index
@@ -1178,7 +1185,7 @@ proc write_update_index {fd pathList totalCnt batch} {
 		close $fd
 		unlock_index
 		if {$update_index_rsd} {
-			show_diff $ui_fname_value
+			reshow_diff
 		} else {
 			set ui_status_value {Ready.}
 		}