Fix error with truncating package name if repo is symlinked

[dritter]

There is a bug with truncate_with_package_name truncation strategy if the repo is symlinked. I encountered this bug, when writing a test for that strategy in #344 , as the temp dir in OSX is actually a symlink to /private/tmp.
The problem is that git rev-parse --show-toplevel always shows the real path on the file system that, if symlinked, is not the root path to the repo. Despite that using git to find the root path of a repository, where potentially a package.json lies is not the best way, this PR just fixes the truncation problem. A better way would be going up a directory, until a package.json is dicovered (similar to @BenoitAverty approach in #353).

In a folder /tmp/repo/1/12/123/1234/12345/123456/1234567/12345678/123456789, with a package.json in /tmp/repo the truncation looked

before: My_Package3/12…/12…/12…/12…/12…/123456789
after: My_Package/1/12/123/12…/12…/12…/12…/12…/123456789

[bhilburn]

Great catch, and nice PR, @dritter! I especially appreciate the good comments =)