build,travis: build only last commit in a branch build

This started out as me trying to fix my Python build
in a local Travis CI env.
After 40+ minutes, the build still did not stop:
https://travis-ci.org/commodo/packages/builds/310413628

There is a bug, where the `.travis_do.sh` script does exit early,
and enters an endless loop, when doing a branch build.
It tries to continuously fetch 50 more revisions.
So, I decided to add some limited support for branch builds.

This also allows a simpler testing with Travis before
opening a PR.

This scope of testing is limited to whatever the last
commit touches, but it's still better than nothing.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

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