Fix Travis PR testing to not overwrite the "master" that Travis creates with our "upstream master", and to also ignore commits with no content (like a Merge commit - there's no need to sign off on literally nothing)

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update REMOTE_TODO.md
Merge pull request #3489 from tianon/travis-fixes

Fix Travis PR testing to not overwrite the "master" that Travis creates with our "upstream master"
Only get rootfs when we need to calculate the image size

Docker-DCO-1.0-Signed-off-by: Michael Crosby <crosby.michael@gmail.com> (github: crosbymichael)
Fix Travis handling of a fully-numeric commit hash being YAML-converted to a number instead of a string

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3491 from tianon/travis-numeric-commit

Fix Travis handling of a fully-numeric commit hash being YAML-converted ...
Merge pull request #3487 from crosbymichael/improve-images-speed

Only get rootfs when we need to calculate the image size