update checkout problem with local branch

@paskino found a problem with our current strategy of using fetch, checkout tag https://github.com/SyneRBI/SyneRBI_VM/blob/7f0749fa1092d50768507b5751a751103550ddce/scripts/UPDATE.sh#L141

See https://github.com/SyneRBI/SyneRBI_VM/pull/151#discussion_r407855603

There are 3 cases:

when using tags like v2.1.0, the checkout will put you on a detached HEAD.
If the "tag" is a remote branchname, e.g. origin/master, then the fetch will have made sure that's up-to-date. So this will be ok presumably. You will also be on a detached HEAD.
If the "tag" is a local branchname, e.g. master, then the checkout doesn't update the local branch with remote items. So this is a problem and needs a pull (actually merge)

However, if you're in a detached HEAD, you cannot pull:

$ git checkout origin/master
$ git pull
You are not currently on a branch.
Please specify which branch you want to merge with.

I guess there is no easy way to check in which of the 3 cases we are. Possibly the easiest thing to do is a git pull anyway after the checkout, and cope with the failure.

I suppose this is why the CMake Externalproject* stuff is so complicated for updating git...

SyneRBI / SyneRBI_VM

update checkout problem with local branch #153