git-gui: Fix applying a line when all following lines are deletions

If a diff looked like:

 @@
  context
 -del1
 -del2

and you wanted to stage the deletion 'del1', the generated patch
wouldn't apply because it was missing the line 'del2' converted to
context, but this line was counted in the @@-line

Signed-off-by: Jeff Epler <jepler@unpythonic.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/diff.tcl

@@ -664,6 +664,7 @@ proc apply_line {x y} {
 		}
 		set i_l $next_l
 	}
+	set patch "$patch$pre_context"
 	set patch "@@ -$hln,$n +$hln,[eval expr $n $sign 1] @@\n$patch"
 
 	if {[catch {