Fix install-doc-quick target

The script starts in a subdirectory of the source directory to
muck with a branch whose structure does not have anything to
do with the actual work tree.  Go up to the top to make it clear
that we operate on the whole tree.

It also exported GIT_DIR without any good reason.  Remove it.

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

Documentation/install-doc-quick.sh

@@ -7,7 +7,7 @@ mandir="$2"
 SUBDIRECTORY_OK=t
 USAGE='<refname> <target directory>'
 . git-sh-setup
-export GIT_DIR
+cd_to_toplevel
 
 test -z "$mandir" && usage
 if ! git rev-parse --verify "$head^0" >/dev/null; then
@@ -18,6 +18,8 @@ fi
 GIT_INDEX_FILE=`pwd`/.quick-doc.index
 export GIT_INDEX_FILE
 rm -f "$GIT_INDEX_FILE"
+trap 'rm -f "$GIT_INDEX_FILE"' 0
+
 git read-tree $head
 git checkout-index -a -f --prefix="$mandir"/