Add possibility to specify refspec instead of branch/tag/etc. for component download.

Since it doesn't matter for git what do you provide branch name, tag name or commit sha - it would be great to specify some kind of refspec for each component (in its configuration).
Refspec can be branch name, tag name, commit sha or even something like 'master@{2 days ago}~6^2~{/Breaking}'. Git downloader should be corrected so it would support refspec. To unify the branch name for each possible refspec during git checkout - <origin>.yaml file name should be used: 'anvil-<origin>' or even '<origin>' (for instance, 'anvil-havana' or 'havana').

This will simplify components download process and resolve such situations when branch and tag are specified at once.