git-gui: add config value gui.diffopts for passing additional diff options

Signed-off-by: NTilman Vogel <tilman.vogel@web.de>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -850,6 +850,7 @@ set default_config(gui.fastcopyblame) false
 set default_config(gui.copyblamethreshold) 40
 set default_config(gui.blamehistoryctx) 7
 set default_config(gui.diffcontext) 5
+set default_config(gui.diffopts) {}
 set default_config(gui.commitmsgwidth) 75
 set default_config(gui.newbranchtemplate) {}
 set default_config(gui.spellingdictionary) {}

lib/diff.tcl

@@ -309,6 +309,7 @@ proc start_show_diff {cont_info {add_opts {}}} {
 
 	lappend cmd -p
 	lappend cmd --color
+	set cmd [concat $cmd $repo_config(gui.diffopts)]
 	if {$repo_config(gui.diffcontext) >= 1} {
 		lappend cmd "-U$repo_config(gui.diffcontext)"
 	}

lib/option.tcl

@@ -153,6 +153,7 @@ proc do_options {} {
 		{i-20..200 gui.copyblamethreshold {mc "Minimum Letters To Blame Copy On"}}
 		{i-0..300 gui.blamehistoryctx {mc "Blame History Context Radius (days)"}}
 		{i-1..99 gui.diffcontext {mc "Number of Diff Context Lines"}}
+		{t gui.diffopts {mc "Additional Diff Parameters"}}
 		{i-0..99 gui.commitmsgwidth {mc "Commit Message Text Width"}}
 		{t gui.newbranchtemplate {mc "New Branch Name Template"}}
 		{c gui.encoding {mc "Default File Contents Encoding"}}