Made make_relative_path handle symlinks correctly.

[christian-oudard]

I ran into this problem when building a relocatable virtualenv on a system with symlinks between the requirements and the virtualenv location. The easy_install.pth file was generated with relative paths that went all the way up to the root and back down again, which broke the relocatability.

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow pip has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master. Unfortunately, this pull request does not cleanly merge against the current master branch.

If you do nothing, this Pull Request will be automatically closed by @BrownTruck since it cannot be merged.

If this pull request is still valid, please rebase it against master (or merge master into it) and resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to rebase/merge and resubmit this Pull Request, here is an example message that you can copy and paste:

I ran into this problem when building a relocatable virtualenv on a system with symlinks between the requirements and the virtualenv location. The easy_install.pth file was generated with relative paths that went all the way up to the root and back down again, which broke the relocatability.

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*This was migrated from pypa/virtualenv#389 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

[BrownTruck]

This Pull Request was closed because it cannot be automatically reparented to the master branch and it appears to have bit rotted.

Please feel free to re-open it or re-submit it if it is still valid and you have rebased it onto master or merged master into it.