gitk: Allow diff view without context lines

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -2146,7 +2146,7 @@ proc makewindow {} {
     label .bleft.mid.labeldiffcontext -text "      [mc "Lines of context"]: "
     pack .bleft.mid.diff .bleft.mid.old .bleft.mid.new -side left
     spinbox .bleft.mid.diffcontext -width 5 -font textfont \
-	-from 1 -increment 1 -to 10000000 \
+	-from 0 -increment 1 -to 10000000 \
 	-validate all -validatecommand "diffcontextvalidate %P" \
 	-textvariable diffcontextstring
     .bleft.mid.diffcontext set $diffcontext
@@ -7308,7 +7308,7 @@ proc diffcontextchange {n1 n2 op} {
     global diffcontextstring diffcontext
 
     if {[string is integer -strict $diffcontextstring]} {
-	if {$diffcontextstring > 0} {
+	if {$diffcontextstring >= 0} {
 	    set diffcontext $diffcontextstring
 	    reselectline
 	}