Fix "make GZ=1 quick-install-doc"

The basic idea is from Mark Levedahl.  I do not use GZ=1 nor
quick-install-doc myself (there obviously is a chicken-and-egg
issue with quick-install-doc for me).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/install-doc-quick.sh

@@ -24,10 +24,8 @@ git read-tree $head
 git checkout-index -a -f --prefix="$mandir"/
 
 if test -n "$GZ"; then
-	cd "$mandir"
-	for i in `git ls-tree -r --name-only $head`
-	do
-		gzip < $i > $i.gz && rm $i
-	done
+	git ls-tree -r --name-only $head |
+	xargs printf "$mandir/%s\n" |
+	xargs gzip -f
 fi
 rm -f "$GIT_INDEX_FILE"