mirror repository URLs should permit dots in repository path

[lelutin]

currently in app/models/repository_mirror.rb the regex used for validating mirror URL doesn't accept dots in the repository path. having dots in dir/file names is a common thing in *nix systems so the mirror URL regex should accept them.

this part of the regex: ([\w\/\-~]+)(\.git)

should be adapted to: ([\w\/\-\.~]+)(\.git)