Bump gitaly-proto version to 0.20.0

See merge request !152

Support unhiding of all refs for Geo Nodes

Closes gitlab-ee#2959

See merge request !150

By default, we hide certain refs (e.g. keep-around/XXX) to save user
download time. We need to preserve these references on secondary Geo
nodes to have a true copy.

Update vendoring for Gitaly

See merge request !148

Handle internal API errors when getting merge request URLs

See merge request !145

Update CHANGELOG

See merge request !144

Gitaly SSH Client

See merge request !139

Revert "Merge branch 'sh-disable-rubygems' into 'master'"

See merge request !143

This reverts commit a18c9012, reversing
changes made to 0a646241.

Disable RubyGems to increase performance

See merge request !141

By default, RubyGems is enabled, which causes the Ruby interpreter
to load 600+ additional files at startup when gitlab-shell is
designed not to use any external gems.