[Install] Fetch all symbolic refs (not just branch heads)

[ConsoleCatzirl]

The intention here is to allow installing a gem from git using a
ref of the form "refs/zuul/" (which are used by the zuul
continuous-integration system). There are likely other use cases for
arbitrary refs.

An example Gemfile that would take advantage of this functionality can
be seen at [1]; however, it currently fails to fetch the existing ref
with the error seen at [2].

[1] https://review.openstack.org/#/c/351437/1/Gemfile
[2] http://logs.openstack.org/37/351437/1/check/gate-puppet-log_processor-puppet-lint/7be9b6a/console.html#_2016-08-05_02_06_48_423939

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This change is 

[segiddins]

Could this have performance implications?

[segiddins]

Also, could you please add a test confirming that non-symbolic refs work? Thanks!

[indirect]

This would (for example) have a dramatic impact on any Github-hosted open-source project, since Github creates a named ref for every single PR that has ever been opened. Pulling all named refs will mean fetching a lot of commits that aren't in any branch in the repo. :/

[ConsoleCatzirl]

I would really like to be able to install gems from arbitrary remote refs in my CI system, but I can't find a way to make this configurable to prevent the performance impact on Github projects.

[segiddins]

I think this could happen if you only fetch the named refs asked for in a gemfile?

[segiddins]

See #5624

[bundlerbot]

☔ The latest upstream changes (presumably #5624) made this pull request unmergeable. Please resolve the merge conflicts.