bundle install - include commit hash for git source

[revett]

When using a :gitsource for a gem, the output when running bundle install does not include the commit hash, simply the branch:

Output

Fetching git@github.com:revett/composition.git
Using composition 0.0.1 from git@github.com:revett/composition.git (at master)

This makes it look as though it's using the latest version of the gem from master. However the Gemfile.lock locks the gem to any previous commit hash (whenever the gem was last updated).

Would be great to have this information in the output - very helpful in the console output of Jenkins CI jobs for example.

Gemfile

source "https://rubygems.org/"

group :production do
  gem "composition", :git => "git@github.com:revett/composition.git"
  gem "faraday"
  gem "mustache", "~> 0.99.5"
end

[revett]

Similiar to #178.