feature: only clone if branch/tag/commit exists

[Z3po]

It would be cool additional to feature #6 bringing linking support to puppet environments to have the possibility to specify in the puppetfile to ignore checkout errors.
That could mean something like

mod 'site',
  :git => 'git@puppet-repo.server.lan:pomailcore/site.git',
  :branch => "mybranch",
  :ignore-unreachable => 'True'

Or combined with the new feature referenced:

mod 'site',
  :git => 'git@puppet-repo.server.lan:pomailcore/site.git',
  :link => 'True',
  :ignore-unreachable => 'True'

That causes all you puppetfiles in all environments to look the very same.
Now if you're working on a module and want to test these changes e.g. in your "QA" stage, you could branch the specific git repository to the branch "QA" and after triggering g10k it's available within the environment, without the need to manually specify any new versions through your Puppetfile.

Now it's not necessary anymore to keep all branches in all GIT Repositories "available" but just create them, once you need them.

[xorpaul]

added ignore-unreachable parameter for GIT module with #14