travis-ci: fix "skip_branch_tip_with_tag()" string comparison

09f5e97 ("travis-ci: skip a branch build if equal tag is present",
2017-09-17) introduced the "skip_branch_tip_with_tag" function with
a broken string comparison. Fix it!

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib-travisci.sh

@@ -14,7 +14,7 @@ skip_branch_tip_with_tag () {
 	# of a tag.
 
 	if TAG=$(git describe --exact-match "$TRAVIS_BRANCH" 2>/dev/null) &&
-		$TAG != $TRAVIS_BRANCH
+		test "$TAG" != "$TRAVIS_BRANCH"
 	then
 		echo "Tip of $TRAVIS_BRANCH is exactly at $TAG"
 		exit 0