ui-log: add <span/> around commit decorations

This helps projects that have a large number of tags to display them all
using custom CSS.

The default stylesheet has not been updated since what is useful for
projects with a lot of tags is not the same as what is useful for
projects with only a small number of decorations per commit.

Suggested-by: Konstantin Ryabitsev <mricon@kernel.org>
Signed-off-by: John Keeping <john@keeping.me.uk>