TreeEntry#path should return a posix path

[mojavelinux]

When I use git ls-tree, regardless of whether I'm on Linux, macOS, or Windows, it always reports posix paths (paths that use forward slash as the directory separator). This makes sense since git is a database of virtual paths which have nothing to do with the filesystem / platform I'm currently on. nodegit, on the other hand, returns platform-specific paths. In other words, on Windows, the TreeEntry#path returns a different value than on Linux. It's even stranger that I request an entry using a posix path, but I get back a platform-specific path.

Here's a sample of what git ls-tree -r HEAD returns on Windows:

100644 blob 3fda23c7b550d75eae3d62e471dd35bbc3d6e667    guides/README.md
100644 blob 7a44d0cf690215933974035542d5af6280294982    guides/cloning/README.md
100644 blob 1a9190ab32758755dc1b964e157d2a44abc512d5    guides/cloning/gh-two-factor/README.md
100644 blob d723e52ccf3569b4b25f25ee5d5f21b7a4138453    guides/cloning/gh-two-factor/index.js
100644 blob f6b7c7a3700b1f29bb40b4a9df01795e3a46f106    guides/cloning/index.js

Here are the paths nodegit returns on Windows when walking the same tree:

guides\README.md
guides\cloning\README.md
guides\cloning\gh-two-factor\README.md
guides\cloning\gh-two-factor\index.js
guides\cloning\index.js

This problem can be easily resolved by using path.posix instead of path in tree_entry.js (and perhaps other places).

If the user really wants the Windows-variant of the path, they can easily get it by passing the value to path.normalize.

[mojavelinux]

PR sent #1434

Any feedback?