t5540/t5541: avoid accessing a bare repository via -C <dir>

In the `test_http_push_nonff` function both of these test scripts
call, there were two Git invocations that assume that bare repositories
will always be discovered when the current working directory is inside
one. This is unlikely to be true forever because at some stage, the
`safe.bareRepository` config is prone to be modified to be safe by
default.

So let's be safe and specify the bare repository explicitly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/lib-httpd.sh

@@ -258,7 +258,7 @@ test_http_push_nonff () {
 
 	test_expect_success 'non-fast-forward push fails' '
 		cd "$REMOTE_REPO" &&
-		HEAD=$(git rev-parse --verify HEAD) &&
+		HEAD=$(git --git-dir=. rev-parse --verify HEAD) &&
 
 		cd "$LOCAL_REPO" &&
 		git checkout $BRANCH &&
@@ -269,7 +269,7 @@ test_http_push_nonff () {
 		(
 			cd "$REMOTE_REPO" &&
 			echo "$HEAD" >expect &&
-			git rev-parse --verify HEAD >actual &&
+			git --git-dir=. rev-parse --verify HEAD >actual &&
 			test_cmp expect actual
 		)
 	'