If branch is unset this is just a nicer way to set a
particular commit. However if both are set, then we verify
that the branch/tag is at that particular commit. This is
a nice way to document that we want to use a particular tag
but still protect against the tag changing or a MITM attack
modifying what that tag means.
Instead of one mega flatpak-builder man page, move the
file format documnentation to its own man page in the
right section, and shorten the flatpak-builder one.
Alexander Larsson