Fix Ruby 2.5 compatibility for diverging counts of branches

!31480 does not fully restore compatibility because another Ruby 2.6
feature besides `Enumerable#filter` was used in commit ca5cd7b7. The use
of `Enumerable#to_h`'s block is now replaced by an explicit
`Enumerable#map` call.

Error message:

	TypeError (wrong element type Gitlab::Git::Branch at 0 (expected array)):
  	app/controllers/projects/branches_controller.rb:53:in `to_h'

See https://bugs.ruby-lang.org/issues/15143.

Fixes #64143.