scripts: "export VAR=VALUE" construct is not portable

Found by check-non-portable-shell.pl
Signed-off-by: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/subtree/t/t7900-subtree.sh

@@ -8,7 +8,8 @@ This test verifies the basic operation of the merge, pull, add
 and split subcommands of git subtree.
 '
 
-export TEST_DIRECTORY=$(pwd)/../../../t
+TEST_DIRECTORY=$(pwd)/../../../t
+export TEST_DIRECTORY
 
 . ../../../t/test-lib.sh
 

git-remote-testgit.sh

@@ -13,7 +13,8 @@ refspec="${GIT_REMOTE_TESTGIT_REFSPEC-$default_refspec}"
 
 test -z "$refspec" && prefix="refs"
 
-export GIT_DIR="$url/.git"
+GIT_DIR="$url/.git"
+export GIT_DIR
 
 mkdir -p "$dir"
 

git-stash.sh

@@ -94,7 +94,8 @@ create_stash () {
 		# ease of unpacking later.
 		u_commit=$(
 			untracked_files | (
-				export GIT_INDEX_FILE="$TMPindex"
+				GIT_INDEX_FILE="$TMPindex" &&
+				export GIT_INDEX_FILE &&
 				rm -f "$TMPindex" &&
 				git update-index -z --add --remove --stdin &&
 				u_tree=$(git write-tree) &&