Limit branch name predictions on merge request dropdown

ZenDesk ticket: https://gitlab.zendesk.com/agent/tickets/56341

Description

When using the WEB UI to set up a merge request I need to fill in 2 branches, source and target. I can type these in or use the provided drop-downs. However the dropdowns are consistently cluttered with irrelevant branch names.

As a target, 99% of the time I want either "master" or "release", and I take good care that those words very rarely appear in other branch names. However typing in these names does not reduce the listed branches to only names with those words.

I had hoped to only have to type in an abbreviation, e.g. "rel" to access the branch, but even typing the full branch name is not very helpful. For example, at the moment, there are only three branches with the word "release", but if I type "release" into the textbox of these dropdowns there are 12 branches listed, and 9 of those branches do not have the word "release" in them, and the branch I'm seeking is always at the end of that list.

/cc @awhildy