git-gui: Change prior tree SHA-1 verification to use git_read

This cat-file was done on maint, where we did not have git_read
available to us.  But here on master we do, so we should make
use of it.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

lib/commit.tcl

@@ -258,7 +258,7 @@ proc commit_committree {fd_wt curHEAD msg} {
 	# -- Verify this wasn't an empty change.
 	#
 	if {$commit_type eq {normal}} {
-		set fd_ot [open "| git cat-file commit $PARENT" r]
+		set fd_ot [git_read cat-file commit $PARENT]
 		fconfigure $fd_ot -encoding binary -translation lf
 		set old_tree [gets $fd_ot]
 		close $fd_ot