Make branch icon stand out more (#20726)

Make branch icon stand out more (#20726)

* Make branch icon stand out more

- Currently the branch icon is "squashed" between the two branch names
and feels a bit "amateur-ish" to my feeling(relative to other UI
elements).
- This patch tries to improve that by making the icon bigger
and by adding some margin to not have a "squashed" icon.
- This patch also includes a "fix", for some reason this symbol is not
centering correctly within the span(or without for that matter), so
simply manually patch this by adding `bottom: 1.px`.

* Use svg

* Apply suggestion

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>