[git/alternates] (GH-173) Use 1.8.7 safe Pathname#to_s

`Pathname#to_path` doesn't exist on Ruby 1.8.7 so we have to use
`#to_s`.
Merge branch 'bug/1.2.x/gh-173-pathname-to_s' into 1.2.x
Merge branch '1.2.x' into 1.3.x