Fix custom name in branch creation for issue in Firefox

What does this MR do?

Fix custom name in branch creation for issue in Firefox

Are there points in the code the reviewer needs to double check?

There is only one other usage of Event.srcElement in our code but it is a fallback for Event.currentTarget, see https://gitlab.com/gitlab-org/gitlab-ce/blob/545489a59d477172b90390394979baec92fb0e6c/vendor/assets/javascripts/Chart.js#L711

The branch name input suffers from input loss but we should tackle that in a separate issue, https://gitlab.com/gitlab-org/gitlab-ce/issues/41688

Why was this MR needed?

In browsers that do not support the non-standard Event.srcElement(like Firefox) a custom branch name in the branch/MR creation dropdown was ignored.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Fixed in release it was introduced
• Tests added for this feature/bug
  • Our tests run in Chrome so things just work ™️
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #41563 (closed)