Removed branch info from splash screen on tagged commits

When checking out a tagged commit, git puts you in a detatched state
where you are not currently on a branch. This looks a bit stupid when
compilling official releases that proclames that they are from
(nobranch) or (detatched from .....). This commit adds a flag to
BUILD_INFO to tell if the current commit is tagged, and ignores branch
information on the splach screen if the commit is tagged.

Fixes #4661