Add GetRefsBenchmark for testing refs lookups

Add GetRefsBenchmark for testing refs lookups

Introduce a new benchmark that shows a typical use-case
of opening a cached repository and fetching one ref from
a repository with a high number of refs.

This specific benchmark is tailored to the Gerrit use-case
of reading frequently individual refs SHA1s and by ref-name
prefix from the All-Users repository.

Include the following variables for the benchmark:
- numBranches (from 100 up to 50000)
- trustFolderStat (true or false)
- useRefTable (true or false)

The benchmark needs to be evaluated on a local high-perf SSD
and on a slower NFS network disk.

Bug: 576165
Change-Id: Ie59a87ca9f0191ff30229688e3430332e2f59a4f