Fix i18n on new branch page

What does this MR do and why?

Page url: http://localhost:3000/<group>/<project>/-/branches/new

When the user switches prefer language to Chinese, the page for creating a branch (/-/branches/new) always displays a prompt in English.

The root cause is that the words in HAML file are hardcoded.

In fact, their translations are already in locale/gitlab.pot.

• New Branch
• Branch name
• Existing branch name, tag, or commit SHA

Screenshots

Before	After

cc JH EM @prajnamas