git-gui: Allow as few as 0 lines of diff context

Johannes Sixt pointed out that dropping to 0 lines of context
does allow the user to get more fine-grained hunk selection,
especially since we don't currently support "highlight and
apply (or revert)".

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

lib/diff.tcl

@@ -145,7 +145,7 @@ proc show_diff {path w {lno {}}} {
 
 	lappend cmd -p
 	lappend cmd --no-color
-	if {$repo_config(gui.diffcontext) > 0} {
+	if {$repo_config(gui.diffcontext) >= 0} {
 		lappend cmd "-U$repo_config(gui.diffcontext)"
 	}
 	if {$w eq $ui_index} {