git-gui: respect commit.gpgsign again

As of v2.9.0, `git commit-tree` no longer heeds the `commit.gpgsign`
config setting. This broke committing with GPG signature in Git GUI.

This fixes https://github.com/git-for-windows/git/issues/850

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lib/commit.tcl

@@ -369,6 +369,9 @@ A rescan will be automatically started now.
 	# -- Create the commit.
 	#
 	set cmd [list commit-tree $tree_id]
+	if {[is_config_true commit.gpgsign]} {
+		lappend cmd -S
+	}
 	foreach p [concat $PARENT $MERGE_HEAD] {
 		lappend cmd -p $p
 	}