tg-export.sh: validate and accept multiple -b options

tg-export.sh: validate and accept multiple -b options

From the description it's unclear how multiple branches are
to be specified.

Previously the code accepted ',' separated branch names, but
only one "-b" option (subsequent "-b" values replaced any previous
"-b" values).

However, "," is a valid character in a Git branch name.

Therefore, correct this by only allowing " " separators, but also
allowing multiple "-b" options each of which may contain zero or
more space-separated branch names.

Finally, validate the given branch names because only TopGit branch
names are actually accepted.  This also has the side benefit that
using the name HEAD to refer to the branch a symbolic-ref HEAD points
to just automatically starts working now.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>