Allow browsing branches that end with '.atom'

## What does this MR do?

1. Simplify the regex capture in the routing for the CommitsController
   to not exclude the '.atom' suffix. That's a perfectly valid git
   branch name, so we shouldn't blow up if we get it.
2. Because Rails now can't automatically detect the request format, add
   some code to do so in `ExtractPath` when there is no path. This means
   that, given branches 'foo' and 'foo.atom', the Atom feed for the
   former is unroutable. To fix this: don't do that! Give the branches
   different names!

## Why was this MR needed?

Creating a branch or tag name ending in '.atom' would cause some 500s on that repo.

## What are the relevant issue numbers?

Closes #21955. Related to !5994.

See merge request !6750