Under the hood, #multi uses #pipelined, and we don't care about the
minor differences that using the #multi transaction approach gets us, so
we pick up about 0.1 sec of performance on large-ish diff collections.
This is a minor improvement, but as this is more or less a straight
drop-in replacement, and we're already workign directly with redis-rb,
it seems worthwhile.

Originally I had hoped to feature-flag the changes to support redis hash
caching, as they seemed small, but of course those changes expanded and
expanded until it became clear it really needed to be extracted into a
new class with a parallel API to keep it clean. h/t to @oswaldo for
pointing out when we'd crossed that threshold!

HSET allows us to use a Redis hash, so we can store the combined diffs
for a given merge request rev in a single key _and_ access individual
file diffs as needed.

Fixed new blob and directory form actions in Vue file listing refactor

See merge request gitlab-org/gitlab!20299

Remove unused store properties

See merge request gitlab-org/gitlab!20273