create repository recognition method,

[lukaszb]

Either utils, or Base repostitory should have a smart repository recognition method.
Needed for get_repo() auto alias filling.
And Would be handy in repo_scanners.

I think it should behave very strict. i.e. It has to be sure that given path is one and only one type of repository if it couldn't determine the proper scm raises an exception. e.x a repository under git&hg at the same time should raise an exception.

based on what hg does i think we should assume the fallowing

if dir contains only .hg dir => mercurial
if dir contains only .git dir => git
if dir contains only .bzr dir => bazar
if dir contains only .svn dir => svn

It should fallow symlinks,and (this one is to be decided) should not recurse into directories

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/33/