git-gui: add configurable tab size to the diff view

For Tk 8.5 the "wordprocessor" mode allows us to get a bit fancy for merge
diffs and intend the tabs by one to compensate for the additional diff
marker at the line start.

The code is heavily based on how gitk handles tabs.

Signed-off-by: Michael Lutz <michi@icosahedron.de>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -912,6 +912,7 @@ set default_config(gui.fontdiff) [font configure font_diff]
 set default_config(gui.maxfilesdisplayed) 5000
 set default_config(gui.usettk) 1
 set default_config(gui.warndetachedcommit) 1
+set default_config(gui.tabsize) 8
 set font_descs {
 	{fontui   font_ui   {mc "Main Font"}}
 	{fontdiff font_diff {mc "Diff/Console Font"}}

lib/diff.tcl

@@ -1,6 +1,19 @@
 # git-gui diff viewer
 # Copyright (C) 2006, 2007 Shawn Pearce
 
+proc apply_tab_size {{firsttab {}}} {
+	global have_tk85 repo_config ui_diff
+
+	set w [font measure font_diff "0"]
+	if {$have_tk85 && $firsttab != 0} {
+		$ui_diff configure -tabs [list [expr {$firsttab * $w}] [expr {($firsttab + $repo_config(gui.tabsize)) * $w}]]
+	} elseif {$have_tk85 || $repo_config(gui.tabsize) != 8} {
+		$ui_diff configure -tabs [expr {$repo_config(gui.tabsize) * $w}]
+	} else {
+		$ui_diff configure -tabs {}
+	}
+}
+
 proc clear_diff {} {
 	global ui_diff current_diff_path current_diff_header
 	global ui_index ui_workdir
@@ -105,6 +118,8 @@ proc show_diff {path w {lno {}} {scroll_pos {}} {callback {}}} {
 
 	set cont_info [list $scroll_pos $callback]
 
+	apply_tab_size 0
+
 	if {[string first {U} $m] >= 0} {
 		merge_load_stages $path [list show_unmerged_diff $cont_info]
 	} elseif {$m eq {_O}} {
@@ -401,7 +416,10 @@ proc read_diff {fd conflict_size cont_info} {
 
 		# -- Automatically detect if this is a 3 way diff.
 		#
-		if {[string match {@@@ *} $line]} {set is_3way_diff 1}
+		if {[string match {@@@ *} $line]} {
+			set is_3way_diff 1
+			apply_tab_size 1
+		}
 
 		if {$::current_diff_inheader} {
 

lib/option.tcl

@@ -161,6 +161,7 @@ proc do_options {} {
 		{b gui.warndetachedcommit {mc "Warn before committing to a detached head"}}
 		{s gui.stageuntracked {mc "Staging of untracked files"} {list "yes" "no" "ask"}}
 		{b gui.displayuntracked {mc "Show untracked files"}}
+		{i-1..99 gui.tabsize {mc "Tab spacing"}}
 		} {
 		set type [lindex $option 0]
 		set name [lindex $option 1]