Allow the use of git's useHttpPath setting

[fcwoknhenuxdfiyv]

This PR modifies composePath to take into account the credential.Path field being populated due to useHttpPath = true in a credential stanze in the git config:

[credential "github.com"]
  useHttpPath = true

It will add the first part of the path making the assumtion that it's a user or organisation:

$ gopass list -f git
git/git.somewhere.com/auser
git/github.com/fcwoknhenuxdfiyv/fcwoknhenuxdfiyv
git/github.com/another_github_org/another_gh_user

[fcwoknhenuxdfiyv]

@dominikschulz I just noticed that the tests are failing for Windows. I imagine it has something to do with the path separator. I have no Windows machines to test on.