travis-ci: skip a branch build if equal tag is present

travis-ci: skip a branch build if equal tag is present

If we push a branch and a tag pointing to the HEAD of this branch,
then Travis CI would run the build twice. This wastes resources and
slows the testing.

Add a function to detect this situation and skip the build the branch
if appropriate. Invoke this function on every build.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>